如何声明包含泛型类型的数组？

Question

Say I have the following type:

public class Field<T>
{
  public string Name { get; set; }
  public T Value { get; set; }
}

How can I declare a variable that will contain an array of such Fields? I tried the following:

var customFields = new Field[]
{
    new Field<string> { Name = "nickname", Value = "Bob" },
    new Field<int> { Name = "age", Value = 42 },
    new Field<DateTime> { Name = "customer_since", Value = new DateTime(2000, 12, 1) }
};

but I get the following compile-time exception:

Using the generic type 'Field' requires 1 type arguments

I also tried var customFields = new Field<object>[] but I get the following errors:

Cannot implicitly convert type 'Field' to 'Field'

Cannot implicitly convert type 'Field' to 'Field'

Cannot implicitly convert type 'Field' to 'Field'

Answer 1

As Matias said it can't be done that way because Field<int> and Field<string> are completely different types. What you can do is:

Create a base Field which is not generic:

public class Field
{
    public string Name { get; set; }
}

public class Field<T> : Field
{
    public T Value { get; set; }
}

And then:

var customFields = new Field[]
{
    new Field<string> { Name = "nickname", Value = "Bob" },
    new Field<int> { Name = "age", Value = 42 },
    new Field<DateTime> { Name = "customer_since", Value = new DateTime(2000, 12, 1) }
};

As Matias commented a downcasting will be needed to get the Value of the different types. If for each different type of Value there is a specific logic to perform, a nice way to handle it will be to use the Factory design pattern. The factory will return for each Field a correct " FieldHandler<T> " that will downcast, do the operation and return what is needed.

Answer 2

Another thing you can do is to simply use the common base class for all of those types - Object.

class Field
{
    public string Name { get; set; }
    public Object Value { get; set; }
}
var customFields = new Field[]
{
    new Field { Name = "nickname ", Value = "Bob" },
    new Field { Name = "age", Value = 42 },
    new Field { Name = "customer_since", Value = new DateTime(2000, 12, 1) }
};

However, this would be offloading some type handling to the consumer of customFields, but should be fine if the Value types associated with a specific Field Name are known.