Question

How can one access model’s validation message through ModelState variable at the controller?

Answer 1

string message = ModelState["SomeProperty"].Errors[0].ErrorMessage;

Of course checks must be done as each of those accessors could simply return null. But if you are absolutely sure that SomeProperty has at least one error message in modelstate it’s OK.

Those checks could of course be encapsulated in an extension method so that in your controller you could simply:

string message = ModelState.GetFirstErrorMessageFor("SomeProperty");

Like this for example:

public static class ModelStateExtensions
{
    public static string GetFirstErrorMessageFor(this ModelStateDictionary modelState, string property)
    {
        ModelState value;
        if (!modelState.TryGetValue(property, out value))
        {
            return string.Empty;
        }
        return (value.Errors.FirstOrDefault() ?? new ModelError("")).ErrorMessage;
    }
}