How to declare a local constant in C#?

In C#, you cannot create a constant that is retrieved from a method.

Edit: dead link http://msdn.microsoft.com/en-us/library/e6w8fe1b(VS.71).aspx

This doc should help: https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/const

A constant expression is an expression that can be fully evaluated at compile time.

I'm not sure why readonly and const didn't work for you since these are the keywords you need. You use const if you have a literal (except for array literals) and readonly otherwise:

public void f()
{
    const int answer = 42;
}

private readonly int[] array = new int[] { 1, 2, 3, 4 };
private readonly DateTime date = DateTime.Now;
public void g()
{
    Console.WriteLine(date.ToString());   
}

readonly only works on class level (that is, you can only apply it to fields). Also as a consequence of const requiring a literal, it's inherently static while a readonly field can be either static or instance.

As of 2018-10-02, it isn't possible to have a readonly local in c#, but there is an open proposal for that feature that has ongoing discussion.

This article provides a useful summary.

In the example you gave, you need to declare the variable as static, because you're initializing it with a method call. If you were initializing with a constant value, like 42, you can use const. For classes, structs and arrays, readonly should work.

The const keyword is used to modify a declaration of a field or local variable.

From MSDN.

Since C# can't enforce "const correctnes" (like c++) anyway, I don't think it's very useful. Since functions are very narrwoly scoped, it is easy not to lose oversight.