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public static bool IsNullOrEmpty(this string s)
{
    return string.IsNullOrEmpty(s);
}

public static bool IsNotNullOrEmpty(this string s)
{
    return !string.IsNullOrEmpty(s);
}

Usage:

string name = "René Hézser";

if (name.IsNotNullOrEmpty())
    Console.WriteLine("The name is " + name);