Table of Contents

BLAZMVVM0004: ViewParameter Attribute Usage

Summary

ViewParameter attributes must be used on properties in ViewModels and have matching Component parameters.

Description

This analyzer ensures that [ViewParameter] attributes are only used on ViewModel properties that have corresponding [Parameter] properties in the associated component. This maintains proper data flow between Views and ViewModels.

Severity

Warning - Incorrect usage may cause runtime binding failures.

Why This Rule Exists

  • Ensures proper parameter binding between View and ViewModel
  • Prevents runtime errors from missing parameter mappings
  • Maintains data flow consistency
  • Validates parameter synchronization

✅ DO: Correct Usage

// Component
@inherits MvvmComponentBase<ProductViewModel>

@code {
    [Parameter]
    public int ProductId { get; set; }

    [Parameter]
    public string Category { get; set; } = string.Empty;
}

// ViewModel
[ViewModelDefinition]
public class ProductViewModel : ViewModelBase
{
    [ViewParameter]
    public int ProductId { get; set; }

    [ViewParameter]
    public string Category { get; set; } = string.Empty;

    protected override async Task OnParametersSetAsync()
    {
        // ProductId and Category are automatically synced from component
        await LoadProductAsync(ProductId, Category);
    }
}

❌ DON'T: Incorrect Usage

// ❌ Wrong: ViewParameter in non-ViewModel class
public class MyService
{
    [ViewParameter]  // Error: Not in a ViewModel
    public int Value { get; set; }
}

// ❌ Wrong: ViewParameter without matching Component parameter
[ViewModelDefinition]
public class ProductViewModel : ViewModelBase
{
    [ViewParameter]  // Error: No matching [Parameter] in component
    public int ProductId { get; set; }
}

public class ProductComponent : MvvmComponentBase<ProductViewModel>
{
    // Missing [Parameter] public int ProductId { get; set; }
}

// ❌ Wrong: Name mismatch
public class ProductComponent : MvvmComponentBase<ProductViewModel>
{
    [Parameter]
    public int Id { get; set; }  // Different name
}

[ViewModelDefinition]
public class ProductViewModel : ViewModelBase
{
    [ViewParameter]
    public int ProductId { get; set; }  // Won't bind - names don't match
}

How to Fix

Add Matching Component Parameter

// Before
[ViewModelDefinition]
public class MyViewModel : ViewModelBase
{
    [ViewParameter]
    public int Value { get; set; }
}

public class MyComponent : MvvmComponentBase<MyViewModel>
{
    // Missing parameter
}

// After
public class MyComponent : MvvmComponentBase<MyViewModel>
{
    [Parameter]
    public int Value { get; set; }
}

Remove ViewParameter from Non-ViewModel

// Before
public class MyService
{
    [ViewParameter]
    public int Value { get; set; }
}

// After
public class MyService
{
    public int Value { get; set; }
}

Benefits

Automatic Synchronization

// Component parameter changes automatically sync to ViewModel
[Parameter]
public int ProductId { get; set; }

// ViewModel receives updated value
[ViewParameter]
public int ProductId { get; set; }

Type Safety

// Both must have matching types
[Parameter]
public int ProductId { get; set; }

[ViewParameter]
public int ProductId { get; set; }  // Same type required

Code Fix

This analyzer provides an automatic code fix that adds the missing [Parameter] property to the component.

Additional Resources

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