C Generics and Type Checking

Question

I have a method that uses an IList lt T gt  as a parameter   I need to check what the type of that T object is and do something based on it   I was trying to use the T value  but the compiler does not not allow it   My solution is the following   private static string BuildClause lt T gt  IList lt T gt  clause        if  clause Count  gt  0                if  clause 0  is int    clause 0  is decimal                         do something                   else if  clause 0  is String                         do something else                   else if         etc for all the types         else                      throw new ApplicationException  Invalid type                         There has to be a better way to do this   Is there some way I can check the type of T that is passed in and then use a switch statement

User · Answer

I hope you find this helpful    typeof IList lt T gt   IsGenericType    true typeof IList lt T gt   GetGenericTypeDefinition      typeof IList lt  gt   typeof IList lt int gt   GetGenericArguments   0     typeof int    https   dotnetfiddle net 5qUZnt

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And  because C  has evolved  you can  now  use pattern matching   private static string BuildClause lt T gt  IList lt T gt  clause        if  clause Count  gt  0                switch  clause 0                         case int x     do something with x  which is an int here                case decimal x     do something with x  which is a decimal here                case string x     do something with x  which is a string here                                default  throw new ApplicationException  Invalid type                        And again with switch expressions in C  8 0  the syntax gets even more succinct   private static string BuildClause lt T gt  IList lt T gt  clause        if  clause Count  gt  0                return clause 0  switch                       int x   gt   some string related to this int               decimal x   gt   some string related to this decimal               string x   gt  x                                   gt  throw new ApplicationException  Invalid type

User · Answer

There is no way to use the switch statement for what you want it to do   The switch statement must be supplied with integral types  which does not include complex types such as a  Type  object  or any other object type for that matter

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You can use typeof T    private static string BuildClause lt T gt  IList lt T gt  clause         Type itemType   typeof T        if itemType    typeof int     itemType    typeof decimal

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How about this                    Checks to see if the value passed is valid               if   TypeDescriptor GetConverter typeof T   IsValid value                                 throw new ArgumentException

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By default know there is not a great way   Awhile back I got frustrated with this and wrote a little utility class that helped out a bit and made the syntax a bit cleaner   Essentially it turns the code into  TypeSwitcher Do clause 0     TypeSwitch Case lt int gt  x   gt            x is an int   TypeSwitch Case lt decimal gt  d   gt           d is a decimal    TypeSwitch Case lt string gt  s   gt            s is a string   Full blog post and details on the implementation are available here   http   blogs msdn com jaredpar archive 2008 05 16 switching-on-types aspx

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You can do typeOf T   but I would double check your method and make sure your not violating single responsability here  This would be a code smell  and that s not to say it shouldn t be done but that you should be cautious    The point of generics is being able to build type-agnostic algorthims were you don t care what the type is or as long as it fits within a certain set of criteria  Your implementation isn t very generic

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For everyone that says checking types and doing something based on the type is not a great idea for generics I sort of agree but I think there could be some circumstances where this perfectly makes sense   For example if you have a class that say is implemented like so  Note  I am not showing everything that this code does for simplicity and have simply cut and pasted into here so it may not build or work as intended like the entire code does but it gets the point across   Also  Unit is an enum     public class FoodCount lt TValue gt    BaseFoodCount       public TValue Value   get  set         public override string ToString                 if  Value is decimal                           Code not cleaned up yet                Some code and values defined in base class              mstrValue   Value ToString                decimal mdecValue              decimal TryParse mstrValue  out mdecValue                mstrValue   decimal Round mdecValue  ToString                 mstrValue   mstrValue   mstrUnitOfMeasurement              return mstrValue                    else                          Simply return a string             string str   Value ToString     mstrUnitOfMeasurement              return str                           public class SaturatedFat   FoodCountWithDailyValue lt decimal gt        public SaturatedFat                 mUnit   Unit g            public class Fiber   FoodCount lt int gt        public Fiber                 mUnit   Unit g           public void DoSomething            nutritionFields SaturatedFat oSatFat   new nutritionFields SaturatedFat            string mstrValueToDisplayPreFormatted  oSatFat ToString        So in summary  I think there are valid reasons why you might want to check to see what type the generic is  in order to do something special

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You could use overloads   public static string BuildClause List lt string gt  l        public static string BuildClause List lt int gt  l        public static string BuildClause lt T gt  List lt T gt  l         Or you could inspect the type of the generic parameter   Type listType   typeof T   if listType    typeof int

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The typeof operator     typeof T        won t work with the c  switch statement   But how about this   The following post contains a static class     Is there a better alternative than this to   39 switch on type  39       that will let you write code like this   TypeSwitch Do      sender      TypeSwitch Case lt Button gt       gt  textBox1 Text    Hit a Button        TypeSwitch Case lt CheckBox gt  x   gt  textBox1 Text    Checkbox is     x Checked       TypeSwitch Default      gt  textBox1 Text    Not sure what is hovered over

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Your construction completely defeats the purpose of a generic method   It s ugly on purpose because there must be a better way to achieve what you re trying to accomplish  although you haven t given us quite enough information to figure out what that is

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