如何定义基本类型的通用类型限制？

Question

I have the following method with generic type:

T GetValue<T>();

I would like to limit T to primitive types such as int, string, float but not class type. I know I can define generic for class type like this:

C GetObject<C>() where C: class;

I am not sure if it is possible for primitive types and how if so.

Answer 1

You can use this to limit it to value types:

where C: struct

You also mention string. Unfortunately, strings won't be allowed as they are not value types.

Answer 2

Actually this does the job to certain extend:

public T Object<T>() where T :
   struct, IComparable, IFormattable, IConvertible, IComparable<T>, IEquatable<T>

To limit to numeric types you can get some useful hints of the following samples defined for the ValueType class

Answer 3

这是你要找的东西：

T GetObject<T>() where T : struct;

Answer 4

There is no generic constraint that matches that set of things cleanly. What is it that you actually want to do? For example, you can hack around it with runtime checks, such as a static ctor (for generic types - not so easy for generic methods)...

However; most times I see this, it is because people want one of:

• to be able to check items for equality: in which case use EqualityComparer<T>.Default
• to be able to compare/sort items: in which case use Comparer<T>.Default
• to be able to perform arithmetic: in which case use MiscUtil 's support for generic operators

Answer 5

What are you actually trying to do in the method? It could be that you actually need C to implement IComparable, or someother interface. In which case you want something like

T GetObject<T> where T: IComparable

Answer 6

I'm under the same need, I want to create a method that should retrieve a List where T should be a primitive type like int, double, decimal, etc...

Based on this Microsoft documentation: https://learn.microsoft.com/en-us/do.net/csharp/language-reference/builtin-types/unmanaged-types

Looks like the right approach is to use

where T : unmanaged

quoting:

A type is an unmanaged type if it's any of the following types:

sbyte, byte, short, ushort, int, uint, long, ulong, char, float, double, decimal, or bool Any enum type Any pointer type Any user-defined struct type that contains fields of unmanaged types only and, in C# 7.3 and earlier, is not a constructed type (a type that includes at least one type argument)

Also important quote:

Beginning with C# 7.3, you can use the unmanaged constraint to specify that a type parameter is a non-pointer, non-nullable unmanaged type.

Beginning with C# 8.0, a constructed struct type that contains fields of unmanaged types only is also unmanaged...

Answer 7

如果您需要可以使用为托管类型保留的语言功能（例如 sizeof 运算符）的类型，请使用“非托管”。

where C: unmanaged