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

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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();
    }
}

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