How do I tell C# what kind of literal number I want?

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If you need to tell C# that you want it to treat a literal as a particular
type of number, you may do so by adding a number type suffix at the end of
the literal you provide. For example:

 
1u; // An unsigned int
1l; // A signed long
1ul; // An unsigned long
1f; // A System.Single floating-point number;
1d; // A System.Double floating-point number
1m; // a System.Decimal floating-point number

This is somewhat important because sometimes you must match a literal to the
signature of something or specify the value to 'defeat' an implicit cast
behavior you don't like. For example, Hashtable names = new Hashtable(100, 0.1);
won't compile because the constructor takes parameters (int, float) and the above is
(int, double). The line should read Hashtable names = new Hashtable(100, 0.1f);

A full listing of the suffixes is in the Grammar portion of the C# specification
(appendix A in the ECMA specification, appendix C in the MS specification). The suffixes are also detailed in the
Literals section of the specification (9.4.4 of the ECMA specification, 2.4.4 of the MS specification).

[Author: Jon Skeet]