Nullable type as a generic parameter possible

Question

I want to do something like this    myYear   record GetValueOrNull lt int  gt   myYear      Notice the nullable type as the generic parameter    Since the GetValueOrNull function could return null my first attempt was this    public static T GetValueOrNull lt T gt  this DbDataRecord reader  string columnName    where T   class       object columnValue   reader columnName        if    columnValue is DBNull                 return  T columnValue            return null      But the error I m getting now is      The type  int   must be a reference type in order to use it as parameter  T  in the generic type or method   Right  Nullable lt int gt  is a struct  So I tried changing the class constraint to a struct constraint  and as a side effect can t return null any more    public static T GetValueOrNull lt T gt  this DbDataRecord reader  string columnName    where T   struct   Now the assignment   myYear   record GetValueOrNull lt int  gt   myYear      Gives the following error      The type  int   must be a non-nullable value type in order to use it as parameter  T  in the generic type or method   Is specifying a nullable type as a generic parameter at all possible

User · Answer

I think you want to handle Reference types and struct types. I use it to convert XML Element strings to a more typed type. You can remove the nullAlternative with reflection. The formatprovider is to handle the culture dependent '.' or ',' separator in e.g. decimals or ints and doubles. This may work:

public T GetValueOrNull<T>(string strElementNameToSearchFor, IFormatProvider provider = null ) 
    {
        IFormatProvider theProvider = provider == null ? Provider : provider;
        XElement elm = GetUniqueXElement(strElementNameToSearchFor);

        if (elm == null)
        {
            object o =  Activator.CreateInstance(typeof(T));
            return (T)o; 
        }
        else
        {
            try
            {
                Type type = typeof(T);
                if (type.IsGenericType &&
                type.GetGenericTypeDefinition() == typeof(Nullable<>).GetGenericTypeDefinition())
                {
                    type = Nullable.GetUnderlyingType(type);
                }
                return (T)Convert.ChangeType(elm.Value, type, theProvider); 
            }
            catch (Exception)
            {
                object o = Activator.CreateInstance(typeof(T));
                return (T)o; 
            }
        }
    }

You can use it like this:

iRes = helper.GetValueOrNull<int?>("top_overrun_length");
Assert.AreEqual(100, iRes);



decimal? dRes = helper.GetValueOrNull<decimal?>("top_overrun_bend_degrees");
Assert.AreEqual(new Decimal(10.1), dRes);

String strRes = helper.GetValueOrNull<String>("top_overrun_bend_degrees");
Assert.AreEqual("10.1", strRes);

User · Answer

I know this is old  but here is another solution   public static bool GetValueOrDefault lt T gt  this SqlDataReader Reader  string ColumnName  out T Result        try               object ColumnValue   Reader ColumnName            Result    ColumnValue  null  amp  amp  ColumnValue    DBNull Value     T ColumnValue   default T            return ColumnValue  null  amp  amp  ColumnValue    DBNull Value            catch                  Possibly an invalid cast          return false            Now  you don t care if T was value or reference type  Only if the function returns true  you have a reasonable value from the database  Usage       decimal Quantity  if  rdr GetValueOrDefault lt decimal gt   YourColumnName   out Quantity            Do something with Quantity     This approach is very similar to int TryParse  123   out MyInt

User · Answer

Incase it helps someone - I have used this before and seems to do what I need it to    public static bool HasValueAndIsNotDefault lt T gt  this T  v      where T   struct       return v HasValue  amp  amp   v Value Equals default T

User · Answer

The shorter way    public static T ValueOrDefault lt T gt  this DataRow reader  string columnName    gt           reader IsNull columnName    default    T  reader columnName     return 0 for int  and null for int

User · Answer

Just do two things to your original code     remove the where constraint  and change the last return from return null to return default T   This way you can return whatever type you want   By the way  you can avoid the use of is by changing your if statement to if  columnValue    DBNull Value

User · Answer

Incase it helps someone - I have used this before and seems to do what I need it to    public static bool HasValueAndIsNotDefault lt T gt  this T  v      where T   struct       return v HasValue  amp  amp   v Value Equals default T

User · Answer

This may be a dead thread  but I tend to use the following   public static T  GetValueOrNull lt T gt  this DbDataRecord reader  string columnName  where T   struct        return reader columnName  as T

User · Answer

Just do two things to your original code     remove the where constraint  and change the last return from return null to return default T   This way you can return whatever type you want   By the way  you can avoid the use of is by changing your if statement to if  columnValue    DBNull Value

User · Answer

I think you want to handle Reference types and struct types. I use it to convert XML Element strings to a more typed type. You can remove the nullAlternative with reflection. The formatprovider is to handle the culture dependent '.' or ',' separator in e.g. decimals or ints and doubles. This may work:

public T GetValueOrNull<T>(string strElementNameToSearchFor, IFormatProvider provider = null ) 
    {
        IFormatProvider theProvider = provider == null ? Provider : provider;
        XElement elm = GetUniqueXElement(strElementNameToSearchFor);

        if (elm == null)
        {
            object o =  Activator.CreateInstance(typeof(T));
            return (T)o; 
        }
        else
        {
            try
            {
                Type type = typeof(T);
                if (type.IsGenericType &&
                type.GetGenericTypeDefinition() == typeof(Nullable<>).GetGenericTypeDefinition())
                {
                    type = Nullable.GetUnderlyingType(type);
                }
                return (T)Convert.ChangeType(elm.Value, type, theProvider); 
            }
            catch (Exception)
            {
                object o = Activator.CreateInstance(typeof(T));
                return (T)o; 
            }
        }
    }

You can use it like this:

iRes = helper.GetValueOrNull<int?>("top_overrun_length");
Assert.AreEqual(100, iRes);



decimal? dRes = helper.GetValueOrNull<decimal?>("top_overrun_bend_degrees");
Assert.AreEqual(new Decimal(10.1), dRes);

String strRes = helper.GetValueOrNull<String>("top_overrun_bend_degrees");
Assert.AreEqual("10.1", strRes);

User · Answer

Just do two things to your original code     remove the where constraint  and change the last return from return null to return default T   This way you can return whatever type you want   By the way  you can avoid the use of is by changing your if statement to if  columnValue    DBNull Value

User · Answer

Change the return type to Nullable lt T gt   and call the method with the non nullable parameter static void Main string   args        int  i   GetValueOrNull lt int gt  null  string Empty       public static Nullable lt T gt  GetValueOrNull lt T gt  DbDataRecord reader  string columnName  where T   struct       object columnValue   reader columnName        if    columnValue is DBNull           return  T columnValue       return null

User · Answer

public static T GetValueOrDefault lt T gt  this IDataRecord rdr  int index        object val   rdr index        if    val is DBNull           return  T val       return default T       Just use it like this   decimal  Quantity   rdr GetValueOrDefault lt decimal  gt  1   string Unit   rdr GetValueOrDefault lt string gt  2

User · Answer

Multiple generic constraints can t be combined in an OR fashion  less restrictive   only in an AND fashion  more restrictive   Meaning that one method can t handle both scenarios  The generic constraints also cannot be used to make a unique signature for the method  so you d have to use 2 separate method names   However  you can use the generic constraints to make sure that the methods are used correctly   In my case  I specifically wanted null to be returned  and never the default value of any possible value types  GetValueOrDefault   bad  GetValueOrNull   good   I used the words  Null  and  Nullable  to distinguish between reference types and value types  And here is an example of a couple extension methods I wrote that compliments the FirstOrDefault method in System Linq Enumerable class       public static TSource FirstOrNull lt TSource gt  this IEnumerable lt TSource gt  source          where TSource  class               if  source    null  return null          var result   source FirstOrDefault         Default for a class is null         return result             public static TSource  FirstOrNullable lt TSource gt  this IEnumerable lt TSource  gt  source          where TSource   struct               if  source    null  return null          var result   source FirstOrDefault         Default for a nullable is null         return result

User · Answer

Disclaimer  This answer works  but is intended for educational purposes only     James Jones  solution is probably the best here and certainly the one I d go with  C  4 0 s dynamic keyword makes this even easier  if less safe  public static dynamic GetNullableValue this IDataRecord record  string columnName      var val   reader columnName      return  val    DBNull Value   null   val      Now you don t need the explicit type hinting on the RHS  int  value   myDataReader GetNullableValue  quot MyColumnName quot     In fact  you don t need it anywhere  var value   myDataReader GetNullableValue  quot MyColumnName quot     value will now be an int  or a string  or whatever type was returned from the DB  The only problem is that this does not prevent you from using non-nullable types on the LHS  in which case you ll get a rather nasty runtime exception like  Microsoft CSharp RuntimeBinder RuntimeBinderException  Cannot convert null to  int  because it is a non-nullable value type  As with all code that uses dynamic  caveat coder

User · Answer

I just encountered the same problem myself         reader  myYear   as int   works and is clean   It works with any type without an issue  If the result is DBNull  it returns null as the conversion fails

User · Answer

Just do two things to your original code     remove the where constraint  and change the last return from return null to return default T   This way you can return whatever type you want   By the way  you can avoid the use of is by changing your if statement to if  columnValue    DBNull Value

User · Answer

The shorter way    public static T ValueOrDefault lt T gt  this DataRow reader  string columnName    gt           reader IsNull columnName    default    T  reader columnName     return 0 for int  and null for int

User · Answer

Just had to do something incredible similar to this  My code   public T IsNull lt T gt  this object value  T nullAlterative        if value    DBNull Value                Type type   typeof T           if  type IsGenericType  amp  amp               type GetGenericTypeDefinition      typeof Nullable lt  gt   GetGenericTypeDefinition                          type   Nullable GetUnderlyingType type                      return  T  type IsEnum   Enum ToObject type  Convert ToInt32 value                 Convert ChangeType value  type              else          return nullAlternative

User · Answer

Change the return type to Nullable lt T gt   and call the method with the non nullable parameter static void Main string   args        int  i   GetValueOrNull lt int gt  null  string Empty       public static Nullable lt T gt  GetValueOrNull lt T gt  DbDataRecord reader  string columnName  where T   struct       object columnValue   reader columnName        if    columnValue is DBNull           return  T columnValue       return null

User · Answer

This may be a dead thread  but I tend to use the following   public static T  GetValueOrNull lt T gt  this DbDataRecord reader  string columnName  where T   struct        return reader columnName  as T

User · Answer

public static T GetValueOrDefault lt T gt  this IDataRecord rdr  int index        object val   rdr index        if    val is DBNull           return  T val       return default T       Just use it like this   decimal  Quantity   rdr GetValueOrDefault lt decimal  gt  1   string Unit   rdr GetValueOrDefault lt string gt  2

User · Answer

Change the return type to Nullable lt T gt   and call the method with the non nullable parameter static void Main string   args        int  i   GetValueOrNull lt int gt  null  string Empty       public static Nullable lt T gt  GetValueOrNull lt T gt  DbDataRecord reader  string columnName  where T   struct       object columnValue   reader columnName        if    columnValue is DBNull           return  T columnValue       return null

User · Answer

Disclaimer  This answer works  but is intended for educational purposes only     James Jones  solution is probably the best here and certainly the one I d go with  C  4 0 s dynamic keyword makes this even easier  if less safe  public static dynamic GetNullableValue this IDataRecord record  string columnName      var val   reader columnName      return  val    DBNull Value   null   val      Now you don t need the explicit type hinting on the RHS  int  value   myDataReader GetNullableValue  quot MyColumnName quot     In fact  you don t need it anywhere  var value   myDataReader GetNullableValue  quot MyColumnName quot     value will now be an int  or a string  or whatever type was returned from the DB  The only problem is that this does not prevent you from using non-nullable types on the LHS  in which case you ll get a rather nasty runtime exception like  Microsoft CSharp RuntimeBinder RuntimeBinderException  Cannot convert null to  int  because it is a non-nullable value type  As with all code that uses dynamic  caveat coder

User · Answer

I know this is old  but here is another solution   public static bool GetValueOrDefault lt T gt  this SqlDataReader Reader  string ColumnName  out T Result        try               object ColumnValue   Reader ColumnName            Result    ColumnValue  null  amp  amp  ColumnValue    DBNull Value     T ColumnValue   default T            return ColumnValue  null  amp  amp  ColumnValue    DBNull Value            catch                  Possibly an invalid cast          return false            Now  you don t care if T was value or reference type  Only if the function returns true  you have a reasonable value from the database  Usage       decimal Quantity  if  rdr GetValueOrDefault lt decimal gt   YourColumnName   out Quantity            Do something with Quantity     This approach is very similar to int TryParse  123   out MyInt

User · Answer

I just encountered the same problem myself         reader  myYear   as int   works and is clean   It works with any type without an issue  If the result is DBNull  it returns null as the conversion fails

User · Answer

Just had to do something incredible similar to this  My code   public T IsNull lt T gt  this object value  T nullAlterative        if value    DBNull Value                Type type   typeof T           if  type IsGenericType  amp  amp               type GetGenericTypeDefinition      typeof Nullable lt  gt   GetGenericTypeDefinition                          type   Nullable GetUnderlyingType type                      return  T  type IsEnum   Enum ToObject type  Convert ToInt32 value                 Convert ChangeType value  type              else          return nullAlternative

User · Answer

Multiple generic constraints can t be combined in an OR fashion  less restrictive   only in an AND fashion  more restrictive   Meaning that one method can t handle both scenarios  The generic constraints also cannot be used to make a unique signature for the method  so you d have to use 2 separate method names   However  you can use the generic constraints to make sure that the methods are used correctly   In my case  I specifically wanted null to be returned  and never the default value of any possible value types  GetValueOrDefault   bad  GetValueOrNull   good   I used the words  Null  and  Nullable  to distinguish between reference types and value types  And here is an example of a couple extension methods I wrote that compliments the FirstOrDefault method in System Linq Enumerable class       public static TSource FirstOrNull lt TSource gt  this IEnumerable lt TSource gt  source          where TSource  class               if  source    null  return null          var result   source FirstOrDefault         Default for a class is null         return result             public static TSource  FirstOrNullable lt TSource gt  this IEnumerable lt TSource  gt  source          where TSource   struct               if  source    null  return null          var result   source FirstOrDefault         Default for a nullable is null         return result

User · Answer

Change the return type to Nullable lt T gt   and call the method with the non nullable parameter static void Main string   args        int  i   GetValueOrNull lt int gt  null  string Empty       public static Nullable lt T gt  GetValueOrNull lt T gt  DbDataRecord reader  string columnName  where T   struct       object columnValue   reader columnName        if    columnValue is DBNull           return  T columnValue       return null

[c#] Nullable type as a generic parameter possible?

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