Issue in C# system.math.pow and SQL-Server Pow function

You can be sure in a right result only if this is integer value. The type System.UInt64 is big enough, just because 57 < 64, but not System.UInt32 (57 > 32).

Using Pow function for powers of 2 is just silly. You need to shift 1 to the left using << operator, but for System.UInt64 this is not so easy because this operator is only defined for int. You can do it very simply using the class System.BitConverter, please see:
http://msdn.microsoft.com/en-us/library/system.bitconverter.aspx[^].

Here is how to get an exact answer:

static System.UInt64 PowerOfTwo(byte power) {
    byte[] bytes = new byte[sizeof(System.UInt64)];
    int bitShift = power % 8;
    int byteIndex = power / 8;
    bytes[byteIndex] = (byte)(1 << bitShift);
    return System.BitConverter.ToUInt64(bytes, 0);
}

Testing… the answer is 144115188075855872; your C# result was correct, SQL Server is a liar!

If you need bigger values, your only chance is the type System.Numerics.BigInteger; please see:
http://msdn.microsoft.com/en-us/library/system.numerics.biginteger.aspx[^].

—SA