How to find sources of .Net external external method

I'm looking at HasFlag , there is a call InternalHasFlag(...)

. How can I see its sources?

The question may be a bit misleading (feel free to fix it).

My original problem comes from trying to make an extension method enum

, here is one

public static Enum Mask(this Enum @this, Enum mask)
{
    if (mask == null)
        throw new ArgumentNullException("mask");
    if (!@this.GetType().IsEquivalentTo(mask.GetType()))
        throw new ArgumentException("Type mismatch", "mask");
    return (Enum)(object)((int)(object)@this & (int)(object)mask);
}

      

        
        
        
      

    

I'm very unsure about the last line and wanted to see how ms-guy does it.