string = alias(String)

1.String和string再c#中的有什么不同？

2.datatype  alias

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1.String和string再c#中的有什么不同？

As others have noted, string is an alias for System.String. They compile to the same code, so at execution time there is no difference whatsoever. This is just one of the aliases in C#. The complete list is:

• object: System.Object
• string: System.String
• bool: System.Boolean
• byte: System.Byte
• sbyte: System.SByte
• short: System.Int16
• ushort: System.UInt16
• int: System.Int32
• uint: System.UInt32
• long: System.Int64
• ulong: System.UInt64
• float: System.Single
• double: System.Double
• decimal: System.Decimal
• char: System.Char

Apart from string, object, the aliases are all to value types. decimal is a value type, but not a primitive type in the CLR. The only primitive type which doesn't have an alias is System.IntPtr.

In the spec, the value type aliases are known as "simple types". Literals can be used for constant values of every simple type; no other value types have literal forms available. (Compare this with VB, which allows DateTimeliterals, and has an alias for it too.)

There is one circumstance in which you have to use the aliases: when explicitly specifying an enum's underlying type. For instance:

public enum Foo : UInt32 {} // Invalid
public enum Bar : uint   {} // Valid

Finally, when it comes to which to use: personally I use the aliases everywhere for the implementation, but the CLR type for any APIs. It really doesn't matter too much which you use in terms of implementation - consistency among your team is nice, but no-one else is going to care. On the other hand, it's genuinely important that if you refer to a type in an API, you do so in a language neutral way. A method called "ReadInt32" is unambiguous, whereas a method called "ReadInt" requires interpretation. The caller could be using a language which defines an "int" alias for Int16, for example. The .NET framework designers have followed this pattern, good examples being in the BitConverter, BinaryReader and Convert classes.

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2.datatype  alias

//C#:
using MyType = System.Int32; //define alias MyType for System.Int32

//Now we can use the alias MyType anywhere in our code instead of actual System.In32 like below
MyType myNumber=12; 
Console.WriteLine(myNumber.ToString()); 

//If we want to change to System.Int64, We will simply change the alias, that’s it.
using MyType = System.Int64;