The modulo operator (%) gives a different result for different .NET versions in C#

Math.Pow works on double-precision floating-point numbers; thus, you shouldn’t expect more than the first 15–17 digits of the result to be accurate:

All floating-point numbers also have a limited number of significant digits, which also determines how accurately a floating-point value approximates a real number. A Double value has up to 15 decimal digits of precision, although a maximum of 17 digits is maintained internally.

However, modulo arithmetic requires all digits to be accurate. In your case, you are computing 49¹⁰³, whose result consists of 175 digits, making the modulo operation meaningless in both your answers.

To work out the correct value, you should use arbitrary-precision arithmetic, as provided by the BigInteger class (introduced in .NET 4.0).

int val = (int)(BigInteger.Pow(49, 103) % 143);   // gives 114

Edit: As pointed out by Mark Peters in the comments below, you should use the BigInteger.ModPow method, which is intended specifically for this kind of operation:

int val = (int)BigInteger.ModPow(49, 103, 143);   // gives 114