Friday, 15 April 2016

Create Custom Data Annotation Validators in MVC

To create a custom data annotation validator follow these gudelines:
1)    Your class has to inherit from System.ComponentModel.DataAnnotations.ValidationAttribute class.
2)    Override bool IsValid(object value) method and implement validation logic inside it.

Sometimes developers check that value is not null/empty and return false. This is usually incorrect behaviour, because that's on Required validator to check which means that your custom validators should only validate non-null data but return true otherwise (see example). This will make them usable on mandatory (required) and non-mandatory fields.
public class StringLengthRangeAttribute : ValidationAttribute
    public int Minimum { get; set; }
    public int Maximum { get; set; }

    public StringLengthRangeAttribute()
        this.Minimum = 0;
        this.Maximum = int.MaxValue;

    public override bool IsValid(object value)
        string strValue = value as string;
        if (!string.IsNullOrEmpty(strValue))
            int len = strValue.Length;
            return len >= this.Minimum && len <= this.Maximum;
        return true;
All properties can be set in attribute as you wish to set them. Some examples:
[StringLengthRange(Minimum = 10, ErrorMessage = "Must be >10 characters.")]

[StringLengthRange(Maximum = 20)]

[StringLengthRange(Minimum = 10, Maximum = 20)]

When a particular property isn't set, its value is set in the constructor, so it always has a value. In above usage examples I deliberately added the Required validator as well, so it's in sync with the above caution I've written.
So this validator will still work on your model value that's not required, but when it's present it validates (think of a text field in a web form, that's not required, but if a user enters a value in, it has to be valid).


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