Type.GenericParameterPosition Property
Definition
Important
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Gets the position of the type parameter in the type parameter list of the generic type or method that declared the parameter, when the Type object represents a type parameter of a generic type or a generic method.
public:
abstract property int GenericParameterPosition { int get(); };public:
virtual property int GenericParameterPosition { int get(); };public abstract int GenericParameterPosition { get; }public virtual int GenericParameterPosition { get; }member this.GenericParameterPosition : intPublic MustOverride ReadOnly Property GenericParameterPosition As IntegerPublic Overridable ReadOnly Property GenericParameterPosition As IntegerProperty Value
The position of a type parameter in the type parameter list of the generic type or method that defines the parameter. Position numbers begin at 0.
Exceptions
The current type does not represent a type parameter. That is, IsGenericParameter returns false.
Examples
The following example defines a generic class with two type parameters and defines a second generic class that derives from the first class. The derived class's base class has two type arguments: the first is Int32, and the second is a type parameter of the derived type. The example displays information about these generic classes, including the positions reported by the GenericParameterPosition property.
using namespace System;
using namespace System::Reflection;
using namespace System::Collections::Generic;
// Define a base class with two type parameters.
generic< class T,class U >
public ref class Base {};
// Define a derived class. The derived class inherits from a constructed
// class that meets the following criteria:
// (1) Its generic type definition is Base<T, U>.
// (2) It specifies int for the first type parameter.
// (3) For the second type parameter, it uses the same type that is used
// for the type parameter of the derived class.
// Thus, the derived class is a generic type with one type parameter, but
// its base class is an open constructed type with one type argument and
// one type parameter.
generic<class V>
public ref class Derived : Base<int,V> {};
public ref class Test
{
public:
static void Main()
{
Console::WriteLine(
L"\r\n--- Display a generic type and the open constructed");
Console::WriteLine(L" type from which it is derived.");
// Create a Type object representing the generic type definition
// for the Derived type. Note the absence of type arguments.
//
Type^ derivedType = Derived::typeid;
DisplayGenericTypeInfo(derivedType);
// Display its open constructed base type.
DisplayGenericTypeInfo(derivedType->BaseType);
}
private:
static void DisplayGenericTypeInfo(Type^ t)
{
Console::WriteLine(L"\r\n{0}", t);
Console::WriteLine(L"\tIs this a generic type definition? {0}",
t->IsGenericTypeDefinition);
Console::WriteLine(L"\tIs it a generic type? {0}", t->IsGenericType);
Console::WriteLine(L"\tDoes it have unassigned generic parameters? {0}",
t->ContainsGenericParameters);
if (t->IsGenericType)
{
// If this is a generic type, display the type arguments.
//
array<Type^>^typeArguments = t->GetGenericArguments();
Console::WriteLine(L"\tList type arguments ({0}):",
typeArguments->Length);
System::Collections::IEnumerator^ myEnum =
typeArguments->GetEnumerator();
while (myEnum->MoveNext())
{
Type^ tParam = safe_cast<Type^>(myEnum->Current);
// IsGenericParameter is true only for generic type
// parameters.
//
if (tParam->IsGenericParameter)
{
Console::WriteLine(
L"\t\t{0} (unassigned - parameter position {1})",
tParam, tParam->GenericParameterPosition);
}
else
{
Console::WriteLine(L"\t\t{0}", tParam);
}
}
}
}
};
int main()
{
Test::Main();
}
/* This example produces the following output:
--- Display a generic type and the open constructed
type from which it is derived.
Derived`1[V]
Is this a generic type definition? True
Is it a generic type? True
Does it have unassigned generic parameters? True
List type arguments (1):
V (unassigned - parameter position 0)
Base`2[System.Int32,V]
Is this a generic type definition? False
Is it a generic type? True
Does it have unassigned generic parameters? True
List type arguments (2):
System.Int32
V (unassigned - parameter position 0)
*/
using System;
using System.Reflection;
using System.Collections.Generic;
// Define a base class with two type parameters.
public class Base<T, U> { }
// Define a derived class. The derived class inherits from a constructed
// class that meets the following criteria:
// (1) Its generic type definition is Base<T, U>.
// (2) It specifies int for the first type parameter.
// (3) For the second type parameter, it uses the same type that is used
// for the type parameter of the derived class.
// Thus, the derived class is a generic type with one type parameter, but
// its base class is an open constructed type with one type argument and
// one type parameter.
public class Derived<V> : Base<int, V> { }
public class Test
{
public static void Main()
{
Console.WriteLine(
"\r\n--- Display a generic type and the open constructed");
Console.WriteLine(" type from which it is derived.");
// Create a Type object representing the generic type definition
// for the Derived type, by omitting the type argument. (For
// types with multiple type parameters, supply the commas but
// omit the type arguments.)
//
Type derivedType = typeof(Derived<>);
DisplayGenericTypeInfo(derivedType);
// Display its open constructed base type.
DisplayGenericTypeInfo(derivedType.BaseType);
}
private static void DisplayGenericTypeInfo(Type t)
{
Console.WriteLine("\r\n{0}", t);
Console.WriteLine("\tIs this a generic type definition? {0}",
t.IsGenericTypeDefinition);
Console.WriteLine("\tIs it a generic type? {0}",
t.IsGenericType);
Console.WriteLine("\tDoes it have unassigned generic parameters? {0}",
t.ContainsGenericParameters);
if (t.IsGenericType)
{
// If this is a generic type, display the type arguments.
//
Type[] typeArguments = t.GetGenericArguments();
Console.WriteLine("\tList type arguments ({0}):",
typeArguments.Length);
foreach (Type tParam in typeArguments)
{
// IsGenericParameter is true only for generic type
// parameters.
//
if (tParam.IsGenericParameter)
{
Console.WriteLine(
"\t\t{0} (unassigned - parameter position {1})",
tParam,
tParam.GenericParameterPosition);
}
else
{
Console.WriteLine("\t\t{0}", tParam);
}
}
}
}
}
/* This example produces the following output:
--- Display a generic type and the open constructed
type from which it is derived.
Derived`1[V]
Is this a generic type definition? True
Is it a generic type? True
Does it have unassigned generic parameters? True
List type arguments (1):
V (unassigned - parameter position 0)
Base`2[System.Int32,V]
Is this a generic type definition? False
Is it a generic type? True
Does it have unassigned generic parameters? True
List type arguments (2):
System.Int32
V (unassigned - parameter position 0)
*/
open System
open System.Reflection
open System.Collections.Generic
// Define a base class with two type parameters.
type Base<'T, 'U>() = class end
// Define a derived class. The derived class inherits from a constructed
// class that meets the following criteria:
// (1) Its generic type definition is Base<T, U>.
// (2) It specifies int for the first type parameter.
// (3) For the second type parameter, it uses the same type that is used
// for the type parameter of the derived class.
// Thus, the derived class is a generic type with one type parameter, but
// its base class is an open constructed type with one type argument and
// one type parameter.
type Derived<'V>() = inherit Base<int, 'V>()
let displayGenericTypeInfo (t: Type) =
printfn $"\n{t}"
printfn $"\tIs this a generic type definition? {t.IsGenericTypeDefinition}"
printfn $"\tIs it a generic type? {t.IsGenericType}"
printfn $"\tDoes it have unassigned generic parameters? {t.ContainsGenericParameters}"
if t.IsGenericType then
// If this is a generic type, display the type arguments.
let typeArguments = t.GetGenericArguments()
printfn $"\tList type arguments ({typeArguments.Length}):"
for tParam in typeArguments do
// IsGenericParameter is true only for generic type
// parameters.
if tParam.IsGenericParameter then
printfn $"\t\t{tParam} (unassigned - parameter position {tParam.GenericParameterPosition})"
else
printfn $"\t\t{tParam}"
printfn $"\r\n--- Display a generic type and the open constructed"
printfn $" type from which it is derived."
// Create a Type object representing the generic type definition
// for the Derived type, by omitting the type argument. (For
// types with multiple type parameters, supply the commas but
// omit the type arguments.)
//
let derivedType = (typeof<Derived<_>>).GetGenericTypeDefinition()
displayGenericTypeInfo derivedType
// Display its open constructed base type.
displayGenericTypeInfo derivedType.BaseType
(* This example produces the following output:
--- Display a generic type and the open constructed
type from which it is derived.
Derived`1[V]
Is this a generic type definition? True
Is it a generic type? True
Does it have unassigned generic parameters? True
List type arguments (1):
V (unassigned - parameter position 0)
Base`2[System.Int32,V]
Is this a generic type definition? False
Is it a generic type? True
Does it have unassigned generic parameters? True
List type arguments (2):
System.Int32
V (unassigned - parameter position 0)
*)
Imports System.Reflection
Imports System.Collections.Generic
' Define a base class with two type parameters.
Public Class Base(Of T, U)
End Class
' Define a derived class. The derived class inherits from a constructed
' class that meets the following criteria:
' (1) Its generic type definition is Base<T, U>.
' (2) It uses int for the first type parameter.
' (3) For the second type parameter, it uses the same type that is used
' for the type parameter of the derived class.
' Thus, the derived class is a generic type with one type parameter, but
' its base class is an open constructed type with one assigned type
' parameter and one unassigned type parameter.
Public Class Derived(Of V)
Inherits Base(Of Integer, V)
End Class
Public Class Test
Public Shared Sub Main()
Console.WriteLine(vbCrLf _
& "--- Display a generic type and the open constructed")
Console.WriteLine(" type from which it is derived.")
' Create a Type object representing the generic type definition
' for the Derived type, by omitting the type argument. (For
' types with multiple type parameters, supply the commas but
' omit the type arguments.)
'
Dim derivedType As Type = GetType(Derived(Of ))
DisplayGenericTypeInfo(derivedType)
' Display its open constructed base type.
DisplayGenericTypeInfo(derivedType.BaseType)
End Sub
Private Shared Sub DisplayGenericTypeInfo(ByVal t As Type)
Console.WriteLine(vbCrLf & "{0}", t)
Console.WriteLine(vbTab & "Is this a generic type definition? " _
& t.IsGenericTypeDefinition)
Console.WriteLine(vbTab & "Is it a generic type? " _
& t.IsGenericType)
Console.WriteLine(vbTab _
& "Does it have unassigned generic parameters? " _
& t.ContainsGenericParameters)
If t.IsGenericType Then
' If this is a generic type, display the type arguments.
'
Dim typeArguments As Type() = t.GetGenericArguments()
Console.WriteLine(vbTab & "List type arguments (" _
& typeArguments.Length & "):")
For Each tParam As Type In typeArguments
' IsGenericParameter is true only for generic type
' parameters.
'
If tParam.IsGenericParameter Then
Console.WriteLine(vbTab & vbTab & tParam.ToString() _
& " (unassigned - parameter position " _
& tParam.GenericParameterPosition & ")")
Else
Console.WriteLine(vbTab & vbTab & tParam.ToString())
End If
Next tParam
End If
End Sub
End Class
' This example produces the following output:
'
'--- Display a generic type and the open constructed
' type from which it is derived.
'
'Derived`1[V]
' Is this a generic type definition? True
' Is it a generic type? True
' Does it have unassigned generic parameters? True
' List type arguments (1):
' V (unassigned - parameter position 0)
'
'Base`2[System.Int32,V]
' Is this a generic type definition? False
' Is it a generic type? True
' Does it have unassigned generic parameters? True
' List type parameters (2):
' System.Int32
' V (unassigned - parameter position 0)
'
Remarks
The GenericParameterPosition property returns the position of a type parameter in the parameter list of the generic type definition or generic method definition where the type parameter was originally defined. The DeclaringType and DeclaringMethod properties identify the generic type or method definition:
If the DeclaringMethod property returns a MethodInfo, that MethodInfo represents a generic method definition, and the current Type object represents a type parameter of that generic method definition.
If the DeclaringMethod property returns
null, then the DeclaringType property always returns a Type object representing a generic type definition, and the current Type object represents a type parameter of that generic type definition.
To provide the correct context for the value of the GenericParameterPosition property, it's necessary to identify the generic type or method a type parameter belongs to. For example, consider the return value of the generic method GetSomething in the following code:
generic<typename T, typename U> public ref class B { };
generic<typename V> public ref class A
{
public:
generic<typename X> B<V, X>^ GetSomething()
{
return gcnew B<V, X>();
}
};
public class B<T, U> { }
public class A<V>
{
public B<V, X> GetSomething<X>()
{
return new B<V, X>();
}
}
type B<'T, 'U>() = class end
type A<'V>() =
member _.GetSomething<'X>() =
B<'V, 'X>()
Public Class B(Of T, U)
End Class
Public Class A(Of V)
Public Function GetSomething(Of X)() As B(Of V, X)
Return New B(Of V, X)()
End Function
End Class
The type returned by GetSomething depends on the type arguments supplied to class A and to GetSomething itself. You can obtain a MethodInfo for GetSomething, and from that you can obtain the return type. When you examine the type parameters of the return type, GenericParameterPosition returns 0 for both. The position of V is 0 because V is the first type parameter in the type parameter list for class A. The position of X is 0 because X is the first type parameter in the type parameter list for GetSomething.
Note
Calling the GenericParameterPosition property causes an exception if the current Type does not represent a type parameter. When you examine the type arguments of an open constructed type, use the IsGenericParameter property to tell which are type parameters and which are types. The IsGenericParameter property returns true for a type parameter; you can then use the GenericParameterPosition method to obtain its position and use the DeclaringMethod and DeclaringType properties to determine the generic method or type definition that defines it.
