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Comparing a Generic Type Variable with null
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Comparing a generic type variable to null by using the == or != operator is legal regardless of whether the generic type is constrained:
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private static void ComparingAGenericTypeVariableWithNull<T>(T obj) { if (obj == null) { /* Never executes for a value type */ } }
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Since T is unconstrained, it could be a reference type or a value type . If T is a value type, obj can never be null . Normally, you d expect the C# compiler to issue an error because of this . However, the C# compiler does not issue an error; instead, it compiles the code just fine . When this method is called using a type argument that is a value type, the JIT compiler sees that the if statement can never be true, and the JIT compiler will not emit the native code for the if test or the code in the braces . If I had used the != operator, the JIT compiler would
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Part II Designing Types
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not emit the code for the if test (since it is always true), and it will emit the code inside the if s braces . By the way, if T had been constrained to a struct, the C# compiler would issue an error because you shouldn t be writing code that compares a value type variable with null since the result is always the same .
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Comparing Two Generic Type Variables with Each Other
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Comparing two variables of the same generic type is illegal if the generic type parameter is not known to be a reference type:
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private static void ComparingTwoGenericTypeVariables<T>(T o1, T o2) { if (o1 == o2) { } // Error }
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In this example, T is unconstrained, and whereas it is legal to compare two reference type variables with one another, it is not legal to compare two value type variables with one another unless the value type overloads the == operator . If T were constrained to class, this code would compile, and the == operator would return true if the variables referred to the same object, checking for exact identity . Note that if T were constrained to a reference type that overloaded the operator == method, the compiler would emit calls to this method when it sees the == operator . Obviously, this whole discussion applies to uses of the != operator too . When you write code to compare the primitive value types Byte, Int32, Single, Decimal, etc . the C# compiler knows how to emit the right code . However, for non-primitive value types, the C# compiler doesn t know how to emit the code to do comparisons . So if ComparingTwoGenericTypeVariables method s T were constrained to struct, the compiler would issue an error . And you re not allowed to constrain a type parameter to a specific value type because it is implicitly sealed, and therefore no types exist that are derived from the value type . Allowing this would make the generic method constrained to a specific type, and the C# compiler doesn t allow this because it is more efficient to just make a non-generic method .
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Using Generic Type Variables as Operands
Finally, it should be noted that there are a lot of issues about using operators with generic type operands . In 5, I talked about C# and how it handles its primitive types: Byte, Int16, Int32, Int64, Decimal, and so on . In particular, I mentioned that C# knows how to interpret operators (such as +, -, *, and /) when applied to the primitive types . Well, these operators can t be applied to variables of a generic type because the compiler doesn t know the type at compile time . This means that you can t use any of these operators with variables of a generic type . So it is impossible to write a mathematical algorithm that works on an arbitrary numeric data type . Here is an example of a generic method that I d like to write:
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