TypeClassParameter

Summary

Replace the parameter of methods or local functions with a type parameter if possible.

Default severity

Warning

Description

The parameter of methods or local functions can be replaced with a type parameter if its type is System.Type and every argument for it is a typeof() operator. For example, the local function Print has a single parameter type, whose type is System.Type, and all the invocations of it are performed with an argument of the typeof() operator whose operand is not a static class, as follows:

public void PrintTypes()
{
    void Print(Type type)
    {
        Console.WriteLine(type.FullName);
    }

    Print(typeof(string));
    Print(typeof(int));
    ⋮

The following code shows the revised version of Print where the parameter type is removed but a type parameter T is added instead:

public void PrintTypes()
{
    void Print<T>()
    {
        var type = typeof(T);
        Console.WriteLine(type.FullName);
    }

    Print<string>();
    Print<int>();
    ⋮

Note that this analyzer doesn't report diagnostics if at least one caller invokes the original version of Print with an argument other than the typeof() operator whose operand is not a static class, because it is unable to replace the parameter type with a type parameter T.

Restriction

This analyzer can only diagnose local functions and private methods with the Visual Studio 2019 editor. To diagnose non-private methods with Visual Studio 2019, perform Build Solution or Analysis ➜ Run Code Analysis.

Code fix

The code fix provides the option of replacing the parameter with a type parameter and inserting a local variable declaration to the top of the method or the local function. The variable name of the inserted declaration is the same as the name of the removed parameter.

Example

Diagnostic

private void DoSomething(Type type)
{
    ⋮
}

public void Invoke()
{
    DoSomething(typeof(string));
}

Code fix

private void DoSomething<T>()
{
    var type = typeof(T);
    ⋮
}

public void Invoke()
{
    DoSomething<string>();
}

Remarks

If a type has both DoSomething<T>() and DoSomething(Type) methods at the same time, the code fix provider renames DoSomething<T> (to DoSomething_0<T>, for example) at first, and then replaces DoSomething(Type) with DoSomething<T>().