如何判断字符串是否为空串？

Code
public sealed class String : IComparable, ICloneable, IConvertible, IEnumerable, IComparable<string>
{
    static String()
    {
        string.Empty = "";

        // Code here
    }

    // Code here

    public static readonly string Empty;

    public static bool operator ==(string a, string b)
    {
        return string.Equals(a, b);
    }

    public static bool Equals(string a, string b)
    {
        if (a == b)
        {
            return true;
        }
        if ((a != null) && (b != null))
        {
            return string.EqualsHelper(a, b);
        }
        return false;
    }

    private static unsafe bool EqualsHelper(string ao, string bo)
    {
        // Code here

        int num1 = ao.Length;
        if (num1 != bo.Length)
        {
            return false;
        }

        // Code here
    }

    private extern int InternalLength();

    public int Length
    {
        get
        {
            return this.InternalLength();
        }
    }
}

public extern bool Equals(String value);