c# string comparison method returning index of first non match

/// <summary>
/// Gets a first different char occurence index
/// </summary>
/// <param name="a">First string</param>
/// <param name="b">Second string</param>
/// <param name="handleLengthDifference">
/// If true will return index of first occurence even strings are of different length
/// and same-length parts are equals otherwise -1
/// </param>
/// <returns>
/// Returns first difference index or -1 if no difference is found
/// </returns>
public int GetFirstBreakIndex(string a, string b, bool handleLengthDifference)
{
    int equalsReturnCode = -1;
    if (String.IsNullOrEmpty(a) || String.IsNullOrEmpty(b))
    {
        return handleLengthDifference ? 0 : equalsReturnCode;
    }

    string longest = b.Length > a.Length ? b : a;
    string shorten = b.Length > a.Length ? a : b;    
    for (int i = 0; i < shorten.Length; i++)
    {
        if (shorten[i] != longest[i])
        {
            return i;
        }
    }

    // Handles cases when length is different (a="1234", b="123")
    // index=3 would be returned for this case
    // If you do not need such behaviour - just remove this
    if (handleLengthDifference && a.Length != b.Length)
    {
        return shorten.Length;
    }

    return equalsReturnCode;
}

If you have .net 4.0 installed, this could be a way:

    string A = "1234567890";
    string B = "1234567880";

    char? firstocurrence = A.Zip(B, (p, q) => new { A = p, B = q })
        .Where(p => p.A != p.B)
        .Select(p => p.A)
        .FirstOrDefault();

edit:

Though, if you need the position:

    int? firstocurrence = A.Zip(B, (p, q) => new { A = p, B = q })
            .Select((p, i) => new { A = p.A, B = p.B, idx = i })
            .Where(p => p.A != p.B)
            .Select(p => p.idx)
            .FirstOrDefault();

An extension method along the lines of the below would do the job:

public static int Your_Name_Here(this string s, string other) 
{
    string first = s.Length < other.Length ? s : other;
    string second = s.Length > other.Length ? s : other;

    for (int counter = 0; counter < first.Length; counter++)
    {
        if (first[counter] != second[counter])
        {
            return counter;
        }
    }
    return -1;
}