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Applying Custom Validation in .NET MAUI DataForm Editors

Updated over 6 months ago

Environment

Version	Product	Author
10.1.0	Telerik UI for .NET MAUI DataForm	Dobrinka Yordanova

This knowledge base article demonstrates how to apply custom validation to the built-in editors of the .NET MAUI DataForm component. Custom validation is essential for enforcing specific rules or conditions that the built-in validation attributes might not cover.

This knowledge base article also answers the following questions:

How can I apply custom validation to a DataForm's editors in .NET MAUI?
What is the procedure for adding custom validation rules to my DataForm ViewModel?
Is there a way to show validation errors at the DataForm level?

Solution

This kb article provides two approaches for applying a custom validation for DataForm editors

Custom Validation on Property Level
Custom Validation on ViewModel Level

Custom Validation on Property Level

To apply a custom validation on a property level, use the CustomValidationAttribute. This attribute can be applied directly to individual properties within your ViewModel.

Here is an example demonstrating how to use CustomValidationAttribute on a property:

csharp

private class ViewModel : NotifyPropertyChangedBase
{
    private DateTime? startDate;
    private DateTime? endDate;

    [Display(Name = "Start Date")]
    [CustomValidation(typeof(ViewModel), nameof(ValidateStartDate))]
    public DateTime? StartDate
    {
        get => this.startDate;
        set => this.UpdateValue(ref this.startDate, value);
    }

    [Display(Name = "End Date")]
    [CustomValidation(typeof(ViewModel), nameof(ValidateEndDate))]
    public DateTime? EndDate
    {
        get => this.endDate;
        set => this.UpdateValue(ref this.endDate, value);
    }

    public static ValidationResult ValidateStartDate(object value, ValidationContext context)
    {
        var startDate = (DateTime?)value;

        if (startDate is null)
        {
            return new ValidationResult("Start Date is required.");
        }

        if (startDate < DateTime.Today)
        {
            return new ValidationResult("Start Date cannot be in the past.");
        }

        return ValidationResult.Success;
    }

    public static ValidationResult ValidateEndDate(object value, ValidationContext context)
    {
        var endDate = (DateTime?)value;

        if (endDate is null)
        {
            return new ValidationResult("End Date is required.");
        }

        if (endDate < DateTime.Today)
        {
            return new ValidationResult("End Date cannot be in the past.");
        }

        return ValidationResult.Success;
    }
}

Custom Validation on ViewModel Level

For validations that involve multiple properties or the entire ViewModel, apply the CustomValidationAttribute at the ViewModel level. This method allows for broader validation scopes, such as comparing two property values:

csharp

[CustomValidation(typeof(ViewModel), nameof(ValidateDates))]
public class ViewModel : NotifyPropertyChangedBase
{
    private DateTime? startDate;
    private DateTime? endDate;

    [Display(Name = "Start Date")]
    public DateTime? StartDate
    {
        get => this.startDate;
        set => this.UpdateValue(ref this.startDate, value);
    }

    [Display(Name = "End Date")]
    public DateTime? EndDate
    {
        get => this.endDate;
        set => this.UpdateValue(ref this.endDate, value);
    }

    public static ValidationResult ValidateDates(object value, ValidationContext context)
    {
        var startDate = (DateTime?)context.Items[nameof(StartDate)];
        var endDate = (DateTime?)context.Items[nameof(EndDate)];

        if (startDate >= endDate)
        {
            return new ValidationResult("Start Date must be before End Date.");
        }

        return ValidationResult.Success;
    }
}

Displaying Validation Errors

To display validation errors at the DataForm level, set the IsValidationSummaryVisible property to True. This configuration ensures that all validation errors are visible to the user:

xaml

<telerik:RadDataForm IsValidationSummaryVisible="True" />

Alternatively, you can define custom validation rules directly in XAML, providing flexibility in specifying validation logic: