ulong

Keyword used for unsigned 64-bit integers. It represents System.UInt64 data type found in mscorlib.dll which is implicitly referenced by every C# project when you create them.

Range: 0 to 18,446,744,073,709,551,615

ulong veryLargeInt = 18446744073609451315;
var anotherVeryLargeInt = 15446744063609451315UL;