WPF Validation - Using IDataErrorInfo

In this blog post, I will show you how you can validate data using IDataErrorInfo. This interface has only two members which need to be implemented:

public string Error { get; } - gets an error message indicating what is wrong with this object
public string this[string columnName] { get; } - get the error message for the property with the given name.

Once you implement the IDataErrorInfo you need to set the ValidatesOnDataErrors to true in the element that you want to validate. To simplify the code example, I implemented IDataErrorInfo in the MainViewModel as you can see below:

public class MainViewModel : BindableBase, IDataErrorInfo
{
    private string _firstName;
    private string _lastName;

    public string FirstName
    {
        get { return _firstName; }
        set { _firstName = value; RaisePropertyChanged(); }
    }              

    public string LastName
    {
        get { return _lastName; }
        set { _lastName = value; RaisePropertyChanged(); }
    }

    public string this[string columnName]
    {
        get
        {
            string error = string.Empty;

            switch (columnName)
            {
                case nameof(FirstName):
                    if (string.IsNullOrWhiteSpace(FirstName))
                        error = "First name cannot be empty.";
                    if (FirstName?.Length > 50)
                        error = "The name must be less than 50 characters.";
                    break;

                case nameof(LastName):
                    if (string.IsNullOrWhiteSpace(LastName))
                        error = "Last name cannot be empty.";
                    break;
            }

            return error;
        }
    }

    public string Error => string.Empty;
}

and in the TextBox element, I set ValidatesOnDataErrors to true:

<TextBox Text="{Binding FirstName, Mode=TwoWay,
            UpdateSourceTrigger=PropertyChanged, ValidatesOnDataErrors=True}" />

If you want to have only information about which property is invalid you don’t have to implement Error property (as I have done in my example). Full example with the IDataErrorInfo implementation you can find under the link: https://github.com/kmatyaszek/WPFValidationIDataErrorInfo

WPF Validation - Using IDataErrorInfo

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