Question

How can I generate random Int64 and UInt64 values using the Random class in C#?

Answer 1

This should do the trick. (It’s an extension method so that you can call it just as you call the normal Next or NextDouble methods on a Random object).

public static Int64 NextInt64(this Random rnd) {     var buffer = new byte[sizeof(Int64)];     rnd.NextBytes(buffer);     return BitConverter.ToInt64(buffer, 0); } 

Just replace Int64 with UInt64 everywhere if you want unsigned integers instead and all should work fine.

Note: Since no context was provided regarding security or the desired randomness of the generated numbers (in fact the OP specifically mentioned the Random class), my example simply deals with the Random class, which is the preferred solution when randomness (often quantified as information entropy) is not an issue. As a matter of interest, see the other answers that mention RNGCryptoServiceProvider (the RNG provided in the System.Security namespace), which can be used almost identically.