Why Generic Method Return Types Cannot Be Inferred in .NET
In .NET, generic methods cannot have their return types inferred. This restriction is enforced to prevent type inference from flowing "both ways" within an expression, which can lead to combinatorial explosions of possible type combinations.
Example
Consider the following generic method:
static TDest Gimme<TSource, TDest>(TSource source)
{
return default(TDest);
}
If return type inference were allowed, the following code would be valid:
string dest = Gimme(5);
However, this code would result in a compiler error because the return type of Gimme cannot be inferred from the argument type int.
Reasoning
The reasoning behind this restriction is to prevent situations where type information flows both from the inside and outside of an expression. Consider the following scenarios:
Scenario 1: Multiple Overloads
Suppose we have ten overloads of a method N with different argument types. If we allowed return type inference for generic methods, we would need to infer the return type of G in the expression N(G(5)). This would require considering all ten overloads of N and selecting the "best" one. However, the criteria for determining the "best" overload would be unclear, leading to potential ambiguities.
Scenario 2: Conditional Expressions
Consider the expression double x = b ? G(5) : 123. If return type inference were allowed, we would need to determine the return type of G based on the type of the conditional expression (double). However, this would not account for the possibility that the return type of G might need to be implicitly convertible to the argument type of the conditional expression (int).
Scenario 3: Nested Expressions
If we combine multiple conditional expressions and method calls, such as in the expression N(N(b ? G(5) * G("hello") : 123)), the complexity of return type inference increases exponentially. We would need to consider all possible overloads of G and N in combination, leading to an explosion of possible type combinations.
Conclusion
By prohibiting return type inference for generic methods, .NET prevents these combinatorial explosions and ensures that type inference flows in a predictable and consistent manner.
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