返回否定的布尔值时，“ Cast是多余的”警告

Question

The following program compiles fine in Visual Studio 2015 Update 2, but a "Cast is redundant" warning is generated:

class Program
{
    static void Main(string[] args)
    {
        Program p = new Program();
        bool negated = p.NegateBool(true);
    }

    public bool NegateBool(object value)
    {
        // "Cast is redundant" warning.
        return !(bool)value;

        // No warning.
        //return (bool)value;

        // No warning.
        //var negated = !(bool)value;
        //return negated;
    }
}

When accepting the "Quick Fix" suggestion to remove the cast, the program no longer compiles.

This is only happening when the cast is done directly in the return statement and the negation operator is used.

If the result is put into a variable and then returned, no warning is generated.

Does anyone know why this is happening?

Answer 1

This was a bug in the Roslyn compiler.

I have filed a bug report on Github and the problem will be fixed in a future version:
https://github.com/dotnet/roslyn/issues/10311

Answer 2

bool negated = p.NegatedBool(true);

compiler already knows that you are passing and boolean at this point. try truee instead of true and it will give you an error. and if you type true compiler knows that its a boolean from system.boolean , just hover your mouse over true. so now the compiler already knows that its an bool. just put a debug point on your methods first line. thats the opening curly brace , and go to debug. if your add a watch on value you will know it may show something like this object{bool} in its type.