Why do I need to use M when declaring a decimal

When I try to declare a variable like this:

decimal order = 5.0;

I get the error message "Literal of type double cannot be implicitly converted to type 'decimal'; use an 'M' suffix to create a liter of this type."

Can someone explain why the M is needed?

Any numeric literal with a decimal point but no suffix is of type double. From the C# 5 specification, section 2.4.4.3:

If no real-type-suffix is specified, the type of the real literal is double. Otherwise, the real type suffix determines the type of the real literal, as follows: [...]

There's no implicit conversion from double to decimal, so trying to assign a double value to a decimal variable fails.

You'd get the same thing if you wanted a float value:

float x = 5.0; // Nope, same problem

You could explicitly cast to float or decimal, but that would be a bad idea for float and a very bad idea for decimal.

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