How to resolve InvalidCastException error in sample code listed in "Use an EventCallback to handle events across components" Blazor training module.

Question

This question is related to the following Learning Module

Answer 1

Thank you for reaching out to the Microsoft Q&A forum. 

To resolve the InvalidCastException error in your Blazor code when using EventCallback across components, follow these steps: 

1.Check Type Consistency: Ensure that the type used in EventCallback matches exactly between the parent and child components. For example, if you're passing a KeyTransformation object, verify it's used consistently in both components without type mismatches. 

2.Namespace and Using Directives: Confirm that the KeyTransformation class is in the correct namespace (WebApplication.Data in your case) and that you're using the correct using directives to access it. 

3.Parameter Names: Verify that the parameter names (OnKeyPressCallback) are consistent between where it's defined (TextDisplay) and where it's used (TextTransformer). They should match exactly. 

4.Method Signatures: Ensure that the method signature in the parent component (TransformText) matches the expected signature in the child component (HandleKeyPress). 

5.Debugging Tools: Utilize debugging tools in Visual Studio to inspect variable values and trace where the type mismatch might be occurring. Look for explicit cast operations or unintended type conversions. 

Here’s a simplified example with corrected usage: 

TextDisplay.razor 

@* TextDisplay component *@
@using WebApplication.Data;
<input @onkeypress="HandleKeyPress" />
@code {
    [Parameter]
    public EventCallback<KeyTransformation> OnKeyPressCallback { get; set; }
    private async Task HandleKeyPress(KeyboardEventArgs e)
    {
        KeyTransformation transformation = new KeyTransformation() { Key = e.Key };
        await OnKeyPressCallback.InvokeAsync(transformation);
    }
}

KeyTransformation.cs 

namespace WebApplication.Data
{
    public class KeyTransformation
    {
        public string Key { get; set; }
        public string TransformedKey { get; set; }
    }
}

TextTransformer.razor 

@page "/texttransformer"
@using WebApplication.Data;
<TextDisplay OnKeyPressCallback="TransformText" />
@code {
    private void TransformText(KeyTransformation transformation)
    {
        transformation.TransformedKey = transformation.Key.ToUpper();
    }
}

Please feel free to contact us if you have any additional questions.  

If you have found the answer provided to be helpful, please click on the "Accept answer/Upvote" button so that it is useful for other members in the Microsoft Q&A community.    

Thank you. 

Answer 2

Resolved by checking the parameter names OnKeyPressCallback are consistent. They are different in the code copied from the webpage as shown in the attached screenshot. Thanks for your help and the webpage needs to be corrected.