Why Can't C# Infer Generic Types in Methods with Constraints?

C# Generic Type Inference: Understanding its Limitations

When using C# generic methods, it is often assumed that the compiler can infer the generic parameters based on the constraints. However, there is a situation where this inference seems difficult to achieve.

Consider the following code:

<code class="language-csharp">interface IQuery<TResult> { }

interface IQueryProcessor
{
    TResult Process<TQuery, TResult>(TQuery query)
        where TQuery : IQuery<TResult>;
}

class SomeQuery : IQuery<string>
{
}

class Test
{
    void Test(IQueryProcessor p)
    {
        var query = new SomeQuery();

        // 编译失败
        p.Process(query);

        // 需要显式指定泛型参数
        p.Process<SomeQuery, string>(query);
    }
}</code>

This code fails to compile because the generic parameters cannot be inferred. The question is: why can't the compiler deduce the correct type?

According to former Microsoft C# chief engineer Eric Lippert, the reason lies in the way C# performs type inference. Inference is done simply by examining the parameters and corresponding formal parameter types. In this case, the compiler lacks sufficient information to infer the generic parameters.

constraints, such as those defined by where TQuery : IQuery<TResult>, are not considered part of the signature and therefore cannot be used directly for inference.

To solve this problem, the generic parameters must be specified explicitly, as shown in the second Process method call. This is consistent with the C# design philosophy that constraints provide additional information but do not change the signature of the method.

In the latest C# version, the rules for applying constraints have been slightly modified. For the most accurate information, it is recommended to consult the official C# documentation or check the actual implementation of C# 7.3 and above.

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