Custom Attribute to Validate Duplicate Entity

This post is for Duplicate Entity check using C# Attributes,(Based on the question which I recently answered)

If you want to check whether any entity present in database or not, so this can be achieved using Custom Attribute which is mentioned below,

I put this as a Class level attribute but you can extend it and make it for Property levels also.

Attribute Class code as goes..
[AttributeUsage(AttributeTargets.Class)]
public class DuplicateUserAttribute : ValidationAttribute
{
private const string _defaultErrorMessage = “user ‘{0}’ Already exist”;

public DuplicateUserAttribute ()
: base(_defaultErrorMessage)
{
}

public override string FormatErrorMessage(string name)
{
return String.Format(CultureInfo.CurrentUICulture, ErrorMessageString);

}

public override bool IsValid(object value)
{
UserEntity NewUser = value as UserEntity;
//Write here logic to validate the user is already exist in database like
context.UserList.Where(u=>u.Name ==NewUser .UserName)
return ;
}
}

Now you can use simply,
[DuplicateUser]
Class User
{
…
}

You need give reference of System.ComponentModel.DataAnnotations.dll.
In System.ComponentModel.DataAnnotations.dll you can find many predefined attributes also like, RequiredAttribute, DataTypeAttribute.

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