我如何使我的财产验证生效？

Question

I'm trying to create some parameter attributes, but they are not running.

Here's some sample code of the attribute:

public abstract class ArgumentValidationAttribute : Attribute
{
    public abstract void Validate(object value, string argumentName);
}

[AttributeUsage(AttributeTargets.Parameter)]
public class NotNullAttribute : ArgumentValidationAttribute
{
    public override void Validate(object value, string argumentName)
    {
        if (value == null)
        {
            throw new ArgumentNullException(argumentName);
        }
    }
}

and its usage:

public async Task Deactivate(int id, [NotNull][MaxLength(25)] string modifiedBy)
{
    // do something
}

However, when I call Deactivate(5, null) the attribute does not fire. I've put breakpoints in my calling method and the attribute itself, but the breakpoint in the attribute never hits.

How do I get the attribute to call the Validate method?

Answer 1

What you are trying to achieve, makes sense. However, how you are trying to achieve will not work, at least the way you intend it to. Remember attributes adds info to the metadata and NOT to the method itself. I'll demonstrate with a basic example of how attributes work.

For the sake of this example, I have simplified your NotNullAttribute to directly derive from Attribute -

[AttributeUsage(AttributeTargets.Parameter)]
public class NotNullAttribute : Attribute
{
    public void Validate(object value, string argumentName)
    {
        if (value == null)
        {
            throw new ArgumentNullException(argumentName);
        }
    }
}

Now consider this TestClass which is partial and have your Deactivate method -

public partial class TestClass
{
    public void Deactivate(int id, [NotNull] string modifiedBy)
    {
        // do something
    }
}

Second part of the partial TestClass is as follows -

public partial class TestClass
{
    public void CallDeactivate(int id, string modifiedBy)
    {
        var classType = typeof(TestClass);
        var deactivateMethod = classType.GetMethod("Deactivate");
        var parameterModifiedBy = deactivateMethod.GetParameters()[1];
        var notNullAttribute = parameterModifiedBy.CustomAttributes.FirstOrDefault();
        if (notNullAttribute.AttributeType == typeof(NotNullAttribute))
        {
            var attrObj = Activator.CreateInstance<NotNullAttribute>();
            attrObj.Validate(modifiedBy, parameterModifiedBy.Name);

            this.Deactivate(id, modifiedBy);
        }
    }
}

Now you don't call Deactivate directly instead you call 'CallDeactivate' -

var testClass = new TestClass();
testClass.CallDeactivate(12, null);

This will work and will throw ArgumentNullException . What CallDeactivate function do is read metadata of Deactivate function parameters and execute any functionality you want your attribute to define. Similar approach is used by EntityFramework to execute Data Contract attributes.

Hope this answers your question.