Generic constraints are used to restrict type parameters to ensure specific behavior or inheritance relationships, while covariation allows subtype conversion. For example, where T : IComparable ensures that T is comparable; covariation such as IEnumerable allows IEnumerable<string> to be converted to IEnumerable<object>, but it is read only and cannot be modified. Common constraints include class, struct, new(), base class and interface, and multiple constraints are separated by commas; covariation requires the out keyword and is only applicable to interfaces and delegates, which is different from inverter (in keyword). Note that covariance does not support classes, cannot be converted at will, and constraints affect flexibility.

The generic constraints and covariation mechanisms of C# are a knowledge point that many developers cannot avoid when they advance. Many people think they are a bit abstract and even confusing when they first come into contact. In fact, as long as you understand the problems they solve, it will be much easier to use.

Why are generic constraints required?

Generics themselves are very flexible, but because they are too flexible, sometimes we want to limit the types passed in to ensure that they have certain behaviors or inherit from a base class. Generic constraints are needed at this time.

To give the simplest example: you wrote a method that you want to compare the types you passed, such as sorting. If no constraint is added, the compiler does not know whether the T passed in supports comparison operations. So you can write this:

 public class MyList<T> where T : IComparable<T>

This means that T must implement IComparable<T> interface. This allows you to call T.CompareTo() method internally with confidence.

Common generic constraints include:

• where T : class ——Only reference type
• where T : struct ——Only value type
• where T : new() ——There must be a parameter constructor
• where T : SomeBaseClass — Must inherit from the specified class
• where T : ISomeInterface — An interface must be implemented

Multiple constraints can exist at the same time, separated by commas. new() is usually put last in sequence because of the syntax requirements.

What does covariance mean?

Covariance sounds very advanced, but in fact it is a kind of "subtype conversion". For example, if we know string is a subclass of object , can I use IEnumerable<string> as IEnumerable<object> ? Not possible by default unless this interface or delegate supports covariance.

In C#, if a generic interface supports covariance, you will see the writing of out T For example:

 public interface IEnumerable<out T>

out T here means that this type of parameter is only used for output (return value) and cannot be passed in as method parameters. The advantage of this is that it allows implicit conversions, such as:

 IEnumerable<string> strings = new List<string>();
IEnumerable<object> objects = strings; // Covariance takes effect

Note: Covariation is allowed only if type safety is guaranteed. In other words, it can only be read but not modified, otherwise it will destroy data consistency.

Common misunderstandings and precautions

1. Covariance only applies to interfaces and delegates
   Classes do not support covariance or inversion. Only interfaces like IEnumerable<out t></out> and IEnumerator<out t></out> are supported.

2. The difference between out and in keywords needs to be clarified

   • out T : covariance, can only be used as return value
   • in T : Inverter, can only be used as input parameters

3. Generic constraints cannot be added casually
   For example, if you add class constraints to T , you cannot pass the structure in. Sometimes, for flexibility, there is no need to add constraints.

4. Covariance is not a universal type conversion
   Although IEnumerable<string> can be used as IEnumerable<object> , it cannot be the other way around, that is a matter of inversion.

Basically that's it. Although generic constraints and covariance may seem a bit complicated, they all exist to solve practical problems: one is to control the type scope and the other is to enhance type compatibility. If you master these two points, the code you write will be safer and more flexible.

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