Creating a custom validation attribute

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Custom validation attributes can be created by deriving from the ValidationAttribute base class, then overriding virtual methods as needed.

[AttributeUsage(AttributeTargets.Property, AllowMultiple = false, Inherited = false)]
public class NotABananaAttribute : ValidationAttribute
    public override bool IsValid(object value)
        var inputValue = value as string;
        var isValid = true;

        if (!string.IsNullOrEmpty(inputValue))
            isValid = inputValue.ToUpperInvariant() != "BANANA";

        return isValid;

This attribute can then be used like this:

public class Model
    [NotABanana(ErrorMessage = "Bananas are not allowed.")]
    public string FavoriteFruit { get; set; }

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