Covariance and Contravariance (C# and Visual Basic)

In C# and Visual Basic, covariance and contravariance enable implicit reference conversion for array types, delegate types, and generic type arguments. Covariance preserves assignment compatibility and contravariance reverses it.

The following code demonstrates the difference between assignment compatibility, covariance, and contravariance.

' Assignment compatibility.  
Dim str As String = "test" 
' An object of a more derived type is assigned to an object of a less derived type.  
Dim obj As Object = str

' Covariance.  
Dim strings As IEnumerable(Of String) = New List(Of String)()
' An object that is instantiated with a more derived type argument  
' is assigned to an object instantiated with a less derived type argument.  
' Assignment compatibility is preserved.  
Dim objects As IEnumerable(Of Object) = strings

' Contravariance.            
' Assume that there is the following method in the class:  
' Shared Sub SetObject(ByVal o As Object) 
' End Sub 
Dim actObject As Action(Of Object) = AddressOf SetObject

' An object that is instantiated with a less derived type argument  
' is assigned to an object instantiated with a more derived type argument.  
' Assignment compatibility is reversed.  
Dim actString As Action(Of String) = actObject

// Assignment compatibility.  
string str = "test";
// An object of a more derived type is assigned to an object of a less derived type.  
object obj = str;

// Covariance. 
IEnumerable<string> strings = new List<string>();
// An object that is instantiated with a more derived type argument  
// is assigned to an object instantiated with a less derived type argument.  
// Assignment compatibility is preserved. 
IEnumerable<object> objects = strings;

// Contravariance.            
// Assume that the following method is in the class:  
// static void SetObject(object o) { } 
Action<object> actObject = SetObject;
// An object that is instantiated with a less derived type argument  
// is assigned to an object instantiated with a more derived type argument.  
// Assignment compatibility is reversed. 
Action<string> actString = actObject;

Covariance for arrays enables implicit conversion of an array of a more derived type to an array of a less derived type. But this operation is not type safe, as shown in the following code example.

Dim array() As Object = New String(10) {}
' The following statement produces a run-time exception. 
' array(0) = 10

object[] array = new String[10];
// The following statement produces a run-time exception. 
// array[0] = 10;

Covariance and contravariance support for method groups allows for matching method signatures with delegate types. This enables you to assign to delegates not only methods that have matching signatures, but also methods that return more derived types (covariance) or that accept parameters that have less derived types (contravariance) than that specified by the delegate type. For more information, see Variance in Delegates (C# and Visual Basic) and Using Variance in Delegates (C# and Visual Basic).

The following code example shows covariance and contravariance support for method groups.

Shared Function GetObject() As Object 
    Return Nothing 
End Function 

Shared Sub SetObject(ByVal obj As Object)
End Sub 

Shared Function GetString() As String 
    Return "" 
End Function 

Shared Sub SetString(ByVal str As String)

End Sub 

Shared Sub Test()
    ' Covariance. A delegate specifies a return type as object, 
    ' but you can assign a method that returns a string. 
    Dim del As Func(Of Object) = AddressOf GetString

    ' Contravariance. A delegate specifies a parameter type as string, 
    ' but you can assign a method that takes an object. 
    Dim del2 As Action(Of String) = AddressOf SetObject
End Sub

static object GetObject() { return null; }
static void SetObject(object obj) { }

static string GetString() { return ""; }
static void SetString(string str) { }

static void Test()
{
    // Covariance. A delegate specifies a return type as object, 
    // but you can assign a method that returns a string.
    Func<object> del = GetString;

    // Contravariance. A delegate specifies a parameter type as string, 
    // but you can assign a method that takes an object.
    Action<string> del2 = SetObject;
}

In .NET Framework 4 and Visual Studio 2010, both C# and Visual Basic support covariance and contravariance in generic interfaces and delegates and allow for implicit conversion of generic type parameters. For more information, see Variance in Generic Interfaces (C# and Visual Basic) and Variance in Delegates (C# and Visual Basic).

The following code example shows implicit reference conversion for generic interfaces.

Dim strings As IEnumerable(Of String) = New List(Of String)
Dim objects As IEnumerable(Of Object) = strings

IEnumerable<String> strings = new List<String>();
IEnumerable<Object> objects = strings;

A generic interface or delegate is called variant if its generic parameters are declared covariant or contravariant. Both C# and Visual Basic enable you to create your own variant interfaces and delegates. For more information, see Creating Variant Generic Interfaces (C# and Visual Basic) and Variance in Delegates (C# and Visual Basic).

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