建立自定義驗證屬性

可以通過從 ValidationAttribute 基類派生，然後根據需要覆蓋 virtual 方法來建立自定義驗證屬性。

[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;
    }
}

然後可以像這樣使用此屬性：

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