Programmatic equivalent of default Type

Question

I m using reflection to loop through a Type s properties and set certain types to their default   Now  I could do a switch on the type and set the default Type  explicitly  but I d rather do it in one line   Is there a programmatic equivalent of default

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Slight adjustments to  Rob Fonseca-Ensor s solution  The following extension method also works on  Net Standard since I use GetRuntimeMethod instead of GetMethod   public static class TypeExtensions       public static object GetDefault this Type t                var defaultValue   typeof TypeExtensions               GetRuntimeMethod nameof GetDefaultGeneric   new Type                     MakeGenericMethod t  Invoke null  null           return defaultValue             public static T GetDefaultGeneric lt T gt                  return default T                and the according unit test for those who care about quality    Fact  public void GetDefaultTest            Arrange     var type   typeof DateTime           Act     var defaultValue   type GetDefault            Assert     defaultValue Should   Be default DateTime

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You can use PropertyInfo SetValue obj  null    If called on a value type it will give you the default  This behavior is documented in  NET 4 0 and in  NET 4 5

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In case of a value type use Activator CreateInstance and it should work fine  When using reference type just return null     public static object GetDefault Type type       if type IsValueType             return Activator CreateInstance type           return null      In the newer version of  net such as  net standard  type IsValueType needs to be written as type GetTypeInfo   IsValueType

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This is optimized Flem s solution   using System Collections Concurrent   namespace System       public static class TypeExtension                 a thread-safe way to hold default instances created at run-time         private static ConcurrentDictionary lt Type  object gt  typeDefaults              new ConcurrentDictionary lt Type  object gt              public static object GetDefaultValue this Type type                        return type IsValueType                  typeDefaults GetOrAdd type  Activator CreateInstance                   null

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Can t find anything simple and elegant just yet  but I have one idea  If you know the type of the property you wish to set  you can write your own default T   There are two  cases - T is a value type  and T is a reference type  You can see this by checking T IsValueType  If T is a reference type  then you can simply set it to null  If T is a value type  then it will have a default parameterless constructor that you can call to get a  blank  value

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I do the same task like this      in MessageHeader     private void SetValuesDefault                MessageHeader header   this                       Framework ObjectPropertyHelper SetPropertiesToDefault lt MessageHeader gt  this           in ObjectPropertyHelper    public static void SetPropertiesToDefault lt T gt  T obj                    Type objectType   typeof T                System Reflection PropertyInfo    props   objectType GetProperties                 foreach  System Reflection PropertyInfo property in props                                if  property CanWrite                                        string propertyName   property Name                      Type propertyType   property PropertyType                       object value   TypeHelper DefaultForType propertyType                       property SetValue obj  value  null                                            in TypeHelper     public static object DefaultForType Type targetType                return targetType IsValueType   Activator CreateInstance targetType    null

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Slight adjustments to  Rob Fonseca-Ensor s solution  The following extension method also works on  Net Standard since I use GetRuntimeMethod instead of GetMethod   public static class TypeExtensions       public static object GetDefault this Type t                var defaultValue   typeof TypeExtensions               GetRuntimeMethod nameof GetDefaultGeneric   new Type                     MakeGenericMethod t  Invoke null  null           return defaultValue             public static T GetDefaultGeneric lt T gt                  return default T                and the according unit test for those who care about quality    Fact  public void GetDefaultTest            Arrange     var type   typeof DateTime           Act     var defaultValue   type GetDefault            Assert     defaultValue Should   Be default DateTime

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Why do you say generics are out of the picture       public static object GetDefault Type t                Func lt object gt  f   GetDefault lt object gt           return f Method GetGenericMethodDefinition   MakeGenericMethod t  Invoke null  null              private static T GetDefault lt T gt                  return default T

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The Expressions can help here       private static Dictionary lt Type  Delegate gt  lambdasMap   new Dictionary lt Type  Delegate gt          private object GetTypedNull Type type                Delegate func          if   lambdasMap TryGetValue type  out func                         var body   Expression Default type               var lambda   Expression Lambda body               func   lambda Compile                lambdasMap type    func                    return func DynamicInvoke            I did not test this snippet  but i think it should produce  typed  nulls for reference types

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I do the same task like this      in MessageHeader     private void SetValuesDefault                MessageHeader header   this                       Framework ObjectPropertyHelper SetPropertiesToDefault lt MessageHeader gt  this           in ObjectPropertyHelper    public static void SetPropertiesToDefault lt T gt  T obj                    Type objectType   typeof T                System Reflection PropertyInfo    props   objectType GetProperties                 foreach  System Reflection PropertyInfo property in props                                if  property CanWrite                                        string propertyName   property Name                      Type propertyType   property PropertyType                       object value   TypeHelper DefaultForType propertyType                       property SetValue obj  value  null                                            in TypeHelper     public static object DefaultForType Type targetType                return targetType IsValueType   Activator CreateInstance targetType    null

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In case of a value type use Activator CreateInstance and it should work fine  When using reference type just return null     public static object GetDefault Type type       if type IsValueType             return Activator CreateInstance type           return null      In the newer version of  net such as  net standard  type IsValueType needs to be written as type GetTypeInfo   IsValueType

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If you re using  NET 4 0 or above and you want a programmatic version that isn t a codification of rules defined outside of code  you can create an Expression  compile and run it on-the-fly   The following extension method will take a Type and get the value returned from default T  through the Default method on the Expression class   public static T GetDefaultValue lt T gt             We want an Func lt T gt  which returns the default         Create that expression here      Expression lt Func lt T gt  gt  e   Expression Lambda lt Func lt T gt  gt              The default value  always get what the  code  tells us          Expression Default typeof T                  Compile and return the value      return e Compile         public static object GetDefaultValue this Type type           Validate parameters      if  type    null  throw new ArgumentNullException  type            We want an Func lt object gt  which returns the default         Create that expression here      Expression lt Func lt object gt  gt  e   Expression Lambda lt Func lt object gt  gt              Have to convert to object          Expression Convert                 The default value  always get what the  code  tells us              Expression Default type   typeof object                           Compile and return the value      return e Compile          You should also cache the above value based on the Type  but be aware if you re calling this for a large number of Type instances  and don t use it constantly  the memory consumed by the cache might outweigh the benefits

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Can t find anything simple and elegant just yet  but I have one idea  If you know the type of the property you wish to set  you can write your own default T   There are two  cases - T is a value type  and T is a reference type  You can see this by checking T IsValueType  If T is a reference type  then you can simply set it to null  If T is a value type  then it will have a default parameterless constructor that you can call to get a  blank  value

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Equivalent to Dror s answer but as an extension method   namespace System       public static class TypeExtensions               public static object Default this Type type                        object output   null               if  type IsValueType                                output   Activator CreateInstance type                              return output

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I do the same task like this      in MessageHeader     private void SetValuesDefault                MessageHeader header   this                       Framework ObjectPropertyHelper SetPropertiesToDefault lt MessageHeader gt  this           in ObjectPropertyHelper    public static void SetPropertiesToDefault lt T gt  T obj                    Type objectType   typeof T                System Reflection PropertyInfo    props   objectType GetProperties                 foreach  System Reflection PropertyInfo property in props                                if  property CanWrite                                        string propertyName   property Name                      Type propertyType   property PropertyType                       object value   TypeHelper DefaultForType propertyType                       property SetValue obj  value  null                                            in TypeHelper     public static object DefaultForType Type targetType                return targetType IsValueType   Activator CreateInstance targetType    null

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This is optimized Flem s solution   using System Collections Concurrent   namespace System       public static class TypeExtension                 a thread-safe way to hold default instances created at run-time         private static ConcurrentDictionary lt Type  object gt  typeDefaults              new ConcurrentDictionary lt Type  object gt              public static object GetDefaultValue this Type type                        return type IsValueType                  typeDefaults GetOrAdd type  Activator CreateInstance                   null

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Why not call the method that returns default T  with reflection   You can use GetDefault of any type with       public object GetDefault Type t                return this GetType   GetMethod  GetDefaultGeneric   MakeGenericMethod t  Invoke this  null              public T GetDefaultGeneric lt T gt                  return default T

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Why do you say generics are out of the picture       public static object GetDefault Type t                Func lt object gt  f   GetDefault lt object gt           return f Method GetGenericMethodDefinition   MakeGenericMethod t  Invoke null  null              private static T GetDefault lt T gt                  return default T

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Why not call the method that returns default T  with reflection   You can use GetDefault of any type with       public object GetDefault Type t                return this GetType   GetMethod  GetDefaultGeneric   MakeGenericMethod t  Invoke this  null              public T GetDefaultGeneric lt T gt                  return default T

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Equivalent to Dror s answer but as an extension method   namespace System       public static class TypeExtensions               public static object Default this Type type                        object output   null               if  type IsValueType                                output   Activator CreateInstance type                              return output

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This should work  Nullable lt T gt  a   new Nullable lt T gt    GetValueOrDefault

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If you re using  NET 4 0 or above and you want a programmatic version that isn t a codification of rules defined outside of code  you can create an Expression  compile and run it on-the-fly   The following extension method will take a Type and get the value returned from default T  through the Default method on the Expression class   public static T GetDefaultValue lt T gt             We want an Func lt T gt  which returns the default         Create that expression here      Expression lt Func lt T gt  gt  e   Expression Lambda lt Func lt T gt  gt              The default value  always get what the  code  tells us          Expression Default typeof T                  Compile and return the value      return e Compile         public static object GetDefaultValue this Type type           Validate parameters      if  type    null  throw new ArgumentNullException  type            We want an Func lt object gt  which returns the default         Create that expression here      Expression lt Func lt object gt  gt  e   Expression Lambda lt Func lt object gt  gt              Have to convert to object          Expression Convert                 The default value  always get what the  code  tells us              Expression Default type   typeof object                           Compile and return the value      return e Compile          You should also cache the above value based on the Type  but be aware if you re calling this for a large number of Type instances  and don t use it constantly  the memory consumed by the cache might outweigh the benefits

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This should work  Nullable lt T gt  a   new Nullable lt T gt    GetValueOrDefault

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You can use PropertyInfo SetValue obj  null    If called on a value type it will give you the default  This behavior is documented in  NET 4 0 and in  NET 4 5

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The chosen answer is a good answer  but be careful with the object returned   string test   null  string test2       if  test is string       Console WriteLine  This will never be hit     if  test2 is string       Console WriteLine  Always hit       Extrapolating     string test   GetDefault typeof string    if  test is string       Console WriteLine  This will never be hit

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The Expressions can help here       private static Dictionary lt Type  Delegate gt  lambdasMap   new Dictionary lt Type  Delegate gt          private object GetTypedNull Type type                Delegate func          if   lambdasMap TryGetValue type  out func                         var body   Expression Default type               var lambda   Expression Lambda body               func   lambda Compile                lambdasMap type    func                    return func DynamicInvoke            I did not test this snippet  but i think it should produce  typed  nulls for reference types

User · Answer

In case of a value type use Activator CreateInstance and it should work fine  When using reference type just return null     public static object GetDefault Type type       if type IsValueType             return Activator CreateInstance type           return null      In the newer version of  net such as  net standard  type IsValueType needs to be written as type GetTypeInfo   IsValueType

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The chosen answer is a good answer  but be careful with the object returned   string test   null  string test2       if  test is string       Console WriteLine  This will never be hit     if  test2 is string       Console WriteLine  Always hit       Extrapolating     string test   GetDefault typeof string    if  test is string       Console WriteLine  This will never be hit

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lt summary gt          returns the default value of a specified type          lt  summary gt           lt param name  type  gt  lt  param gt      public static object GetDefault this Type type                return type IsValueType     type IsGenericType   Activator CreateInstance type    type GenericTypeArguments 0  GetDefault       null

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Can t find anything simple and elegant just yet  but I have one idea  If you know the type of the property you wish to set  you can write your own default T   There are two  cases - T is a value type  and T is a reference type  You can see this by checking T IsValueType  If T is a reference type  then you can simply set it to null  If T is a value type  then it will have a default parameterless constructor that you can call to get a  blank  value

User · Answer

I do the same task like this      in MessageHeader     private void SetValuesDefault                MessageHeader header   this                       Framework ObjectPropertyHelper SetPropertiesToDefault lt MessageHeader gt  this           in ObjectPropertyHelper    public static void SetPropertiesToDefault lt T gt  T obj                    Type objectType   typeof T                System Reflection PropertyInfo    props   objectType GetProperties                 foreach  System Reflection PropertyInfo property in props                                if  property CanWrite                                        string propertyName   property Name                      Type propertyType   property PropertyType                       object value   TypeHelper DefaultForType propertyType                       property SetValue obj  value  null                                            in TypeHelper     public static object DefaultForType Type targetType                return targetType IsValueType   Activator CreateInstance targetType    null

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lt summary gt          returns the default value of a specified type          lt  summary gt           lt param name  type  gt  lt  param gt      public static object GetDefault this Type type                return type IsValueType     type IsGenericType   Activator CreateInstance type    type GenericTypeArguments 0  GetDefault       null

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Can t find anything simple and elegant just yet  but I have one idea  If you know the type of the property you wish to set  you can write your own default T   There are two  cases - T is a value type  and T is a reference type  You can see this by checking T IsValueType  If T is a reference type  then you can simply set it to null  If T is a value type  then it will have a default parameterless constructor that you can call to get a  blank  value

User · Answer

In case of a value type use Activator CreateInstance and it should work fine  When using reference type just return null     public static object GetDefault Type type       if type IsValueType             return Activator CreateInstance type           return null      In the newer version of  net such as  net standard  type IsValueType needs to be written as type GetTypeInfo   IsValueType

[c#] Programmatic equivalent of default(Type)

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