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I have a webservice which has a method that returns a List of Payment objects provided with some input parameters. However if the input parameters are not in the correct format, I would like to return an error message which is of type string, not a List of Payment objects. I would like do this using Generic Classes concept in C#. Anyone has any idea about how I can manage to do this ?

Here is some code:
        public List<Payment> GetPayments(string firstDate, string lastDate, string entegrationStatus)
            if (Common.IsDateTime(firstDate) && Common.IsDateTime(firstDate) && Common.IsValidEntegrationStatus(entegrationStatus))
                return paymentManager.GetPayments(firstDate, lastDate, entegrationStatus);
                return "Error Message";

Many thanks,
Updated 24-Feb-11 9:03am

When we are unsure about what Type a function will return, we should use Object in that scenario.
you can achieve this without generic. look at the following code.

protected void Page_Load(object sender, EventArgs e)
   // method call
    object ret = method(0);
    if (ret.GetType() == typeof(string))
        Response.Write("String Object Error");

        Response.Write("List Object");

// magic method declaration ..
 object method(int input)
     if (input == 0)
         return new List<int>();
         return "Error in input";

you may use polymorphism concept also. you make return type of Type Base and return the derived types as well. test the type and do accordingly.


Return types are pre-defined at the time of declaration of the Method. you cannot change its Type at run-time using the same function and within the function. return Type can be changed at run-time but not from within the Method.

In generic the return Type is passed along with the Method call like below.
T method<t>(int a)
   T t1
   return t1;

bool c = method<bool>(1);

here the return type is passed when we are calling the method. we cannot decide the Type T from with in the method like .
method(int a)
  return "ass";
  return new List<payment>();

Not a good idea. Is this WCF? If so, the error should be communicated back via an exception. Take a look at the FaultException class.


This is in response to your comment. You could try something like this:

public class Payment
    public int Cost { get; set; }

    public string Detail { get; set; }

public class Payments
    public string ErrorMessage { get; set; }

    public List<Payment> PaymentItems { get; set; }

public Payments GetPayments(int x)
    Payments payments = new Payments();

    if (x == 0)
        payments.ErrorMessage = "An error has occured";
        payments.PaymentItems = new[] { new Payment() { Cost = 99, Detail = "Apples" } }.ToList();
    return payments;

Here's the normal output:


And here's what you get with the error message set:

<ErrorMessage>An error has occured</ErrorMessage>

[Foot Note]

Remember that generic types will not work with web-services unless they are instantiated. You will see this message if you attempt to do that : Only instantiated generic types can be serialized
Close Network 24-Feb-11 13:49pm
Not it is not WCF. I know there are other ways to do this, I want to get a taste of using a generic method/class.
Nish Nishant 24-Feb-11 13:50pm
What does it use then? Is this a regular ASP.NET web service?
Close Network 24-Feb-11 13:59pm
Yes, regular web service.
Nish Nishant 24-Feb-11 14:47pm
Check my updated answer for one way to do this.
Close Network 24-Feb-11 15:10pm
Yes, this is an acceptable solution. And also it would be very nice If could do that with generics.
Nish Nishant 24-Feb-11 15:12pm
Yes, but generic types will not work with web-services unless they are instantiated. You will see this message if you attempt to do that : "Only instantiated generic types can be serialized"
Using generics, you can substitute data types that your method acts acts on. It wont fit your problem.
Here are a few alternatives
1. Return a type of Object. So that you may return a List on success and null on error.
Object o = MyFunction();
   // error
  // typecast object to your list & use

2. If you wish to return different error strings, Return a string & pass out the list from a parameter
private String Func(out List<yourobj> lst)</yourobj>

So that on error the function will return an error description & on success an empty string would be returned along with the list
Nish Nishant 24-Feb-11 13:53pm
Keep in mind that this is a web service.
Probably, you don't really need generics in this simple case; classic OOP might be enough. All types are derived from System.Object so they all has the virtual method System.Object.ToString. You can override this method in any class to represent its content in string form better.

In case of error, you can return something like

string.Format("Invalid input parameters: {0}", GetParameterInfo());

The method GetParameterInfo can traverse all problematic instances and collect the information is some list of strings based on the return of ToString for each.

I realize your case could me more complex, so this simple approach would be not enough. In this case you should share more of your code.


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