How do I handle invalid conversions in MvvmCross

Understanding the Issue

In MvvmCross, when you bind a UI control to a view model property, it attempts to convert the data types automatically. However, if the conversion fails (e.g., trying to convert a string like "22ab" to an integer), MvvmCross does not currently provide a built-in mechanism to notify the view model of this failure directly through the binding layer[1]. Instead, you need to implement custom validation or conversion logic.

Implementing Custom Validation

1. Use String Properties in ViewModel: For free text input, define the view model properties as strings. This allows any input to be accepted without immediate conversion errors.

2. Trigger Validation on Property Change: Implement validation rules in the view model that are triggered when the string property changes. This can involve checking if the string can be converted to the desired type (e.g., integer) and setting an error message if it cannot.

3. Use Value Converters: While value converters are primarily used for converting data types for display, you can also use them to handle invalid conversions by catching exceptions or returning specific values to indicate errors[9].

Handling Conversion Errors

1. Manual Conversion Checks: Before attempting to convert a string to another type, manually check if the conversion is possible. For example, use `int.TryParse` to check if a string can be converted to an integer.

2. Error Messaging: If a conversion fails, set an error message in the view model that can be bound to a UI element to inform the user of the issue.

3. Custom Controls: If necessary, create custom controls that handle input validation at the UI level, such as using `textFieldShouldEndEditing` or `ShouldChangeCharacters` on iOS to validate input before it reaches the view model[1].

Example Implementation

Here's a basic example of how you might handle invalid conversions in a view model:

csharp
public class MyViewModel : MvxViewModel
{
    private string _inputValue;
    public string InputValue
    {
        get => _inputValue;
        set
        {
            _inputValue = value;
            ValidateInput();
            RaisePropertyChanged(() => InputValue);
        }
    }

    private string _errorMessage;
    public string ErrorMessage
    {
        get => _errorMessage;
        set
        {
            _errorMessage = value;
            RaisePropertyChanged(() => ErrorMessage);
        }
    }

    private void ValidateInput()
    {
        if (int.TryParse(InputValue, out int result))
        {
            // Conversion successful, you can use 'result'
            ErrorMessage = null;
        }
        else
        {
            // Conversion failed, set error message
            ErrorMessage = "Invalid input. Please enter a valid integer.";
        }
    }
}

In this example, `InputValue` is a string property that triggers validation when it changes. If the input cannot be converted to an integer, an error message is displayed.

Conclusion

Handling invalid conversions in MvvmCross requires a proactive approach to validation and error handling. By using string properties, implementing custom validation logic, and leveraging value converters, you can effectively manage and communicate conversion errors to the user.