将泛型引入到从非泛型类继承的构造函数中

Question

Question:

Is there a work around to be able to use generics in the constructor of a class that inherits from a class that doesn't have generic types defined?

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Explanation:

I am creating a new custom attribute inheriting from the ValidationAttribute class. I want to use generic types in the constructor as seen below:

DateValidationAttribute : ValidationAttribute {

   public void DateValidationAttribute <TData>(Expression<Func<TData, DateTime?>> property){ ... }

}

However, this doesn't seem to be allowed as the TData and TProperty generic types aren't recognized as generics. In addition I cannot define the generics in the class definition as it inherits from the ValidationAttribute class. The error I get is that TData and TProperty is not defined.

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Goal:

The goal of passing and expression down to have a strongly typed way of passing attribute names. I am writing this attribute to compare two properties of a class. The implementation below is what it looks like. Beneath that is how I want the implementation to look like:

Current:

public DateTime? ToDate { get; set; }

[DateValidation("ToDate")]
public DateTime? FromDate { get; set; }

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Goal:

public DateTime? ToDate { get; set; }

[DateValidation(prop => prop.ToDate)]
public DateTime? FromDate { get; set; }    

Answer 1

The non-support for generics in attributes is the least of your problems. Attributes can only have a specific subset of types (primitives, enums, String, Type and single-dimensional arrays) as constructor arguments , not lambdas. So even if you could use generics you could not do what you're trying to do.

Consider using a string as constructor parameter and using nameof() , eg:

[DateValidation(nameof(ToDate))]

This approach keeps the names in sync with the real property name and thus you get the safety you want when refactoring etc.