Do not declare static members on generic types

Cause

An externally visible generic type contains a static (Shared in Visual Basic) member.

Rule Description

When a static member of a generic type is called, the type argument must be specified for the type. When a generic instance member that does not support inference is called, the type argument must be specified for the member. The syntax for specifying the type argument in these two cases is different and easily confused, as the following calls demonstrate:

' Shared method in a generic type.
GenericType(Of Integer).SharedMethod()

' Generic instance method that does not support inference.
someObject.GenericMethod(Of Integer)()

// Static method in a generic type.
GenericType<int>.StaticMethod();

// Generic instance method that does not support inference.
someObject.GenericMethod<int>();

Generally, both of the prior declarations should be avoided so that the type argument does not have to be specified when the member is called. This results in a syntax for calling members in generics that is no different from the syntax for non-generics. For more information, see Generic methods should provide type parameter.

How to Fix Violations

To fix a violation of this rule, remove the static member or change it to an instance member.

When to Suppress Warnings

Do not suppress a warning from this rule. Providing generics in a syntax that is easy to understand and use reduces the time that is required to learn and increases the adoption rate of new libraries.

Example

The following example shows a method that causes this violation.

Imports System
Imports System.Runtime.InteropServices

Namespace Samples

    Public NotInheritable Class EnumParser(Of T)

        Private Sub New()
        End Sub 

        ' Fires this violation         
        Public Shared Function TryParse(ByVal value As String, <Out()> ByRef result As T) As Boolean 

            Try
                result = DirectCast([Enum].Parse(GetType(T), value), T)
                Return True 
            Catch ex As ArgumentException
            End Try

            result = Nothing 
            Return False 

        End Function 

    End Class 

    Module Program

        Public Sub Main()

            Dim result As DayOfWeek
            ' Must specify type argument             
            If EnumParser(Of DayOfWeek).TryParse("Monday", result) Then
                Console.WriteLine("Conversion Succeeded!")
            End If 

        End Sub 

    End Module 

End Namespace

using System;

namespace Samples
{    
    public static class EnumParser<T>    
    {        // Fires this violation         
        public static bool TryParse(string value, out T result)        
        {            
            try            
            {                
                result = (T)Enum.Parse(typeof(T), value);                
                return true;            
            }            
            catch (ArgumentException)            
            {            
            }
                result = default(T);            
            return false;        
        }    
    }

    static class Program    
    {        
        public static void Main()        
        {            
            DayOfWeek dayOfWeek;            
            // Must specify type argument             
            if (EnumParser<DayOfWeek>.TryParse("Monday", out dayOfWeek))            
            {                
                Console.WriteLine("Conversion Succeeded!");            
            }        
        }    
    }
}

In the example above, declaring a static member on a generic type forces the user to specify the type argument when calling it.

The following example fixes the above violation by moving the type parameter, T, from the class to the method, allowing it to be inferred by the compiler when called.

Related Rules

Avoid excessive parameters on generic types

Collections should implement generic interface

Do not expose generic lists

Do not nest generic types in member signatures

Generic methods should provide type parameter

Use generic event handler instances

Use generics where appropriate

See Also

Reference

Generics (C# Programming Guide)