What is the best way to combine two uints into a ulong in c#

ulong mixed = (ulong)high << 32 | low;

The cast is very important. If you omit the cast, considering the fact that high is of type uint (which is 32 bits), you'll be shifting a 32 bit value 32 bits to the left. Shift operators on 32 bit variables will use shift stuff by right-hand-side mod 32. Effectively, shifting a uint 32 bits to the left is a no-op. Casting to ulong prevents this.

Verifying this fact is easy:

uint test = 1u;
Console.WriteLine(test << 32); // prints 1
Console.WriteLine((ulong)test << 32); // prints (ulong)uint.MaxValue + 1