How do I implement IDataErrorInfo in MvvmCross

Step 1: Define Your ViewModel

First, create a view model that implements `IDataErrorInfo`. This interface requires two properties: `Error` and an indexer that takes a property name as a string.

csharp
using System.ComponentModel;

public class MyViewModel : MvxViewModel, IDataErrorInfo
{
    // Example property
    private string _name;
    public string Name
    {
        get => _name;
        set => SetProperty(ref _name, value);
    }

    // IDataErrorInfo Implementation
    public string Error => string.Empty; // Typically left empty unless you want to provide a general error message

    public string this[string columnName]
    {
        get
        {
            // Implement validation logic here
            switch (columnName)
            {
                case nameof(Name):
                    if (string.IsNullOrEmpty(Name))
                        return "Name cannot be empty.";
                    break;
            }
            return null; // No error
        }
    }
}

Step 2: Implement Validation Logic

In the indexer, you can implement validation logic for each property. The example above checks if the `Name` property is empty. You can extend this to include more complex validation rules.

Step 3: Use Validation in Your View

To display validation errors in your view, you need to bind your controls to the view model properties and enable data error validation. However, since MvvmCross does not directly support `IDataErrorInfo` for cross-platform validation, you might need to use platform-specific code to display validation errors.

For example, in a WPF application, you would set `ValidatesOnDataErrors=True` in your XAML bindings:

xml

Step 4: Handle Validation Errors

To visually indicate validation errors, you can use platform-specific features. For instance, in WPF, you can use a `Style` or `Validation.ErrorTemplate` to display error messages.

Step 5: Cross-Platform Considerations

Since MvvmCross is designed for cross-platform development, you might want to consider using a more universal validation approach that works across all platforms. One strategy is to implement validation logic in your view models and then use platform-specific code to display validation errors.

For example, you could create a base view model class that contains validation logic and then use platform-specific adapters to display errors:

csharp
public abstract class BaseViewModel : MvxViewModel
{
    public abstract bool IsValid { get; }
    public abstract string GetValidationError(string propertyName);
}

public class MyViewModel : BaseViewModel
{
    public override bool IsValid => !string.IsNullOrEmpty(Name);

    public override string GetValidationError(string propertyName)
    {
        switch (propertyName)
        {
            case nameof(Name):
                if (string.IsNullOrEmpty(Name))
                    return "Name cannot be empty.";
                break;
        }
        return null;
    }
}

Then, in your platform-specific code, you can handle displaying these errors.

Conclusion

Implementing `IDataErrorInfo` in MvvmCross is feasible but requires careful consideration of cross-platform compatibility. While you can use `IDataErrorInfo` for validation logic, displaying validation errors may require platform-specific code. A more universal approach involves implementing validation logic in your view models and using platform-specific adapters to display errors.