hi. as you may know we have bool and Boolean  in c# that work completely the same. or single and float accept same values with same range and.......  is there any  reason for it? 

Types like System.Float and System.Boolean come from the CTS (Common Type System), part of the CLI (Common Language Infrastructure) standard. This standard is defined in ECMA-335. They have nothing, per se, to do with C#.

C# is a language generally built on top of the CLI specification; it's standard (defined in ECMA-334) requires that a certain minimal subset of CLI be exposed through. That includes exposing the CTS types. The language then provides the common C-like aliases "bool" and "float" and such for aesthetics or convenience.

2 hours ago, jpetrie said:

requires that a certain minimal subset of CLI be exposed through. That includes exposing the CTS types

thanks a lot for your answer but can you tell me why its required?

By requiring it, the standard is ensuring that C# can be compiled to an intermediate representation any conforming CLI implementation can run, and also that it can inter-operate easily with other such languages (VB.NET, for example). This ability to inter-operate was a key design goal of the platform at the time; supporting it eases adoption and thus increases how quickly the ecosystem will spread.

I encourage you to read some of those linked standards (the introduction material in particular). They're considerably easier to digest than, say, the C++ standard.

The ones with capital letters are blittable types. I can't go more into depth, but maybe, just maybe it's related that in C# the booleans, shorts, and bytes are represented with 4 bytes if on the stack, or maybe it was just the bools... and I don't know if it applies to structs on the stack. 
https://docs.microsoft.com/en-us/dotnet/framework/interop/blittable-and-non-blittable-types

On 3/17/2018 at 3:25 PM, RnzCpp said:

The ones with capital letters are blittable types.

This is not accurate. There is no difference between `System.Boolean` and `bool`. The latter is simply the C# language alias for the former, and there is no functional difference in their usage. I don't think there's anything dictating how much space primitive types take on the stack -- that is an implementation detail and likely depends on the runtime and optimization level being used.

Traditionally, `System.Boolean` is not considered a blittable type because the Win32 boolean type is 32-bits, whereas the .NET representation is 8-bytes. This behavior probably made sense when .NET was first designed, primarily for Win32 systems.