使用泛型而不是为AsEnumerable进行强制转换

Question

I am new to generics in C# and while reading a book stumbled upon an example:

var cars = from car in data.AsEnumerable()
  where
    car.Field<string>("Color") == "Red"
    select new
    {
      ID = car.Field<int>("CarID"),
      Make = car.Field<string>("Make")
    };

The author says that car.Field<string>("Color") gives the additional compile-time checking comparing to (string)car["Color"] . But how does the compiler know that car.Field<string>("Color") is compilable for "Color" and not for "CarID"? Or there is some kind of another "additional compile-time checking" that I miss?

Answer 1

It doesn't give you any additional compile-time checking. If you use the wrong type, in both cases you'll get an exception during run-time.

But it can be useful to do additional stuff that simple cast can't. For example Field<int>("CarId") could call a method that converts the string in the field to an int .

And assuming you're talking about DataRow.Field<T>() , then, according to the documentation, it's useful mostly for dealing with null values and nullable types correctly.

Answer 2

The compiler doesn't know, your specifying that field "Color" is of type string. Internally, the Field<T>() method does it's magic to make that happen.

If you perform a cast ( (string)car["Color"] ) you could encounter a runtime exception if the field value cannot be converted to the destination type.

From memory, if you specify car.Field<string>("ColorID") , you will be able to safely convert int to string without any issues.

Answer 3

Specifically, the real benefit of car.Field<string>("Color") is that it encapsulates testing field values for equality with DBNull.Value , making your code cleaner and easier to read.

In your example, if the value of the "Color" field is null, the Field<T> extension method will return null , while car["Color"] will return DBNull.Value . You can't cast DBNull.Value to string , so the expression (string)car["Color"] raises an InvalidCastException in that case.

Before the development of the DataSetExtensions class, you would need a somewhat verbose expression to assign the value of that field to a string variable:

var color = DBNull.Value.Equals(car["Color"]) ? null : (string)car["Color"];

Another benefit of Field<T> is, as svick notes, that it enables you to work with nullable types and reference types using the same syntax.