Phil's Notes

Mixing Generics in C#

c#

Recently I've come across a situation where I wanted to keep a list of validation errors for fields with different data types. Upon writing this article, however, I realized that I could have simplified the entire process by keeping the expected and actual values all as strings. Since I went through the trouble of going through this, though, I will share what I learned in case it might help in a different situation.

First, we have to create a validation error type. In order to be able to create a list of them, though, you will need to create an interface so that they don't all need to have the same data type.

public interface IValidationError
{
   string FieldName { get; set; }
   Type Type { get; }
   object ExpectedValue { get; set; }
   object ActualValue { get; set; }
}

public class ValidationError<T> : IValidationError
{
   public string FieldName { get; set; } = string.Empty;
   Type IValidationError.Type => typeof(T);
   public T ExpectedValue { get; set; }
   public T ActualValue { get; set; }
   object IValidationError.ExpectedValue 
   { 
      get => ExpectedValue.ToString(); 
      set => ExpectedValue = (T)value; 
   }
   object IValidationError.ActualValue 
   { 
      get => ActualValue.ToString(); 
      set => ActualValue = (T)value; 
   }
}

Now you should be able to use the new class to create a list like so:

List<IValidationError> validationErrors = new();

validationErrors.Add(new ValidationError<string>()
{
      FieldName = "Reference",
      ExpectedValue = "A26",
      ActualValue = "A33"
});

validationErrors.Add(new ValidationError<decimal>()
{
      FieldName = "Cost",
      ExpectedValue = 9.99m,
      ActualValue = 10.99m
});

To get the values and use them:

string errorMessageFormat = "{0} was supposed to be {1} but is instead {2}. ";
string errorMessage = "";

if (validationErrors.Count > 0)
{
      foreach (var error in validationErrors)
      {
         errorMessage += string.Format(errorMessageFormat, error.FieldName, error.ExpectedValue, error.ActualValue);
      }

      Console.WriteLine(errorMessage.Trim());
}

Here's the whole example:

using System;

namespace MixingGenerics
{
    internal class Program
    {
        static void Main()
        {
            List<IValidationError> validationErrors = new();

            validationErrors.Add(new ValidationError<string>()
            {
                FieldName = "Reference",
                ExpectedValue = "A26",
                ActualValue = "A33"
            });

            validationErrors.Add(new ValidationError<decimal>()
            {
                FieldName = "Cost",
                ExpectedValue = 9.99m,
                ActualValue = 10.99m
            });

            string errorMessageFormat = "{0} was supposed to be {1} but is instead {2}. ";
            string errorMessage = "";

            if (validationErrors.Count > 0)
            {
                foreach (var error in validationErrors)
                {
                    errorMessage += string.Format(errorMessageFormat, error.FieldName, error.ExpectedValue, error.ActualValue);
                }

                Console.WriteLine(errorMessage.Trim());
            }
        }
    }

    public interface IValidationError
    {
        string FieldName { get; set; }
        Type Type { get; }
        object ExpectedValue { get; set; }
        object ActualValue { get; set; }
    }

    public class ValidationError<T> : IValidationError
    {
        public string FieldName { get; set; } = string.Empty;
        Type IValidationError.Type => typeof(T);
        public T ExpectedValue { get; set; }
        public T ActualValue { get; set; }
        object IValidationError.ExpectedValue 
        { 
           get => ExpectedValue.ToString(); 
           set => ExpectedValue = (T)value; 
        }
        object IValidationError.ActualValue 
        { 
           get => ActualValue.ToString(); 
           set => ActualValue = (T)value; 
        }
    }
}