Why do some C# lambda expressions compile to static methods?

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This is most likely because there are no closures, for example:

int age = 25;
Action<string> withClosure = s => Console.WriteLine("My name is {0} and I am {1} years old", s, age);
Action<string> withoutClosure = s => Console.WriteLine("My name is {0}", s);
Console.WriteLine(withClosure.Method.IsStatic);
Console.WriteLine(withoutClosure.Method.IsStatic);

This will output false for withClosure and true for withoutClosure.

When you use a lambda expression, the compiler creates a little class to contain your method, this would compile to something like the following (the actual implementation most likely varies slightly):

private class <Main>b__0
{
    public int age;
    public void withClosure(string s)
    {
        Console.WriteLine("My name is {0} and I am {1} years old", s, age)
    }
}

private static class <Main>b__1
{
    public static void withoutClosure(string s)
    {
        Console.WriteLine("My name is {0}", s)
    }
}

public static void Main()
{
    var b__0 = new <Main>b__0();
    b__0.age = 25;
    Action<string> withClosure = b__0.withClosure;
    Action<string> withoutClosure = <Main>b__1.withoutClosure;
    Console.WriteLine(withClosure.Method.IsStatic);
    Console.WriteLine(withoutClosure.Method.IsStatic);
}

You can see the resulting Action<string> instances actually point to methods on these generated classes.

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