EN 08_diagnostics_reference

08. Diagnostics reference

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This document provides a comprehensive list of diagnostic errors emitted by the DependencyPropertyGenerator during source code analysis, along with detailed troubleshooting guidelines. It details the cause of each error and provides concrete code examples to help you quickly resolve build issues.

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Diagnostic IDs quick reference

Diagnostic ID	Severity	Title	Overview
DPG0000	Error	Framework is not recognized	The generator cannot automatically detect the target UI framework.
DPG0001	Error	OnChanged Method Not Found	The specified callback method is missing or has an invalid signature.
DPG0002	Error	Invalid Type Modifier	You cannot apply the generator to classes with the file scope modifier.
DPG0003	Error	Invalid Property Type	You cannot use ref struct types as DependencyProperties.
DPG0004	Error	Reference Type Sharing	Prevents sharing reference type instances across all control instances.
DPG0005	Error	Invalid Callback Signature	The callback requests the old value on a platform that does not support it.
DPG0007	Error	Unsupported Callback Signature	An auto-discovered callback method has an invalid signature.
DPG0008	Error	Invalid Default Expression	The Roslyn parser cannot parse the C# string in DefaultValueExpression.
DPG0009	Info	Duplicate Attribute Helper	Suppresses CS0436 duplicate attribute helper warnings.
DPG9999	Error	Unhandled Exception	An unexpected internal generator exception occurred.

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Error details and solutions

DPG0000: Framework is not recognized

The generator cannot automatically detect the target UI framework (WPF, WinUI, Uno, Avalonia, or MAUI) from your project references.

❌ Cause: Your project (.csproj) lacks the required UI framework packages (for example, Avalonia or Microsoft.WindowsAppSDK), or you are using the generator in a pure class library without defining the target platform.

✅ Solution: Install the necessary NuGet packages or explicitly define the target framework compiler constant in your library.

<!-- Example: Explicitly defining a compiler constant in .csproj -->
<PropertyGroup>
    <DefineConstants>$(DefineConstants);HAS_WPF</DefineConstants>
</PropertyGroup>

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DPG0001: OnChanged Method Not Found or Unsupported

The method specified in the OnChanged attribute argument either does not exist in the class or has an unsupported signature.

❌ Incorrect code:

[DependencyProperty<int>("Count", OnChanged = nameof(OnCountChanged))]
public partial class MyControl : UserControl
{
    // Error: The first argument must be the containing class type (MyControl).
    private void OnCountChanged(int oldValue, int newValue)
    {
    }
}

✅ Correct code:

[DependencyProperty<int>("Count", OnChanged = nameof(OnCountChanged))]
public partial class MyControl : UserControl
{
    // Solution: Add the containing class type as the first argument.
    private void OnCountChanged(MyControl sender, int oldValue, int newValue)
    {
    }
}

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DPG0002: Invalid Type Modifier (File scoped)

You applied the generator to a class using the C# 11 file scope modifier. Roslyn Source Generators cannot generate code for file-scoped types.

❌ Incorrect code:

[DependencyProperty<string>("Text")]
file partial class LocalControl : UserControl // Error: file scoped
{
}

✅ Correct code:

[DependencyProperty<string>("Text")]
internal partial class LocalControl : UserControl // Solution: Use internal or public
{
}

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DPG0003: Invalid Property Type (Ref struct)

ref struct types (such as ReadOnlySpan<T>) cannot reside on the managed heap. Consequently, you cannot use them as a DependencyProperty type, which relies on boxing or object dictionaries.

❌ Incorrect code:

// Error: ReadOnlySpan<char> cannot be boxed.
[DependencyProperty<ReadOnlySpan<char>>("Buffer")]
public partial class MyControl : UserControl
{
}

✅ Correct code:

// Solution: Use a normal struct, array, or Memory<T>.
[DependencyProperty<ReadOnlyMemory<char>>("Buffer")]
public partial class MyControl : UserControl
{
}

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DPG0004: Reference Type Default Value Sharing

You assigned an instance of a reference type (such as a class or List<T>) directly to DefaultValue. In frameworks like WPF, all control instances share reference type default values, causing memory leaks and shared-state bugs. The generator strictly blocks this to prevent such issues.

❌ Incorrect code:

// Error: A single List instance will be shared by all MyControl instances.
[DependencyProperty<List<string>>("Items", DefaultValueExpression = "new()")]
public partial class MyControl : UserControl
{
}

✅ Correct code:

// Solution: Use CreateDefaultValueCallback = true.
[DependencyProperty<List<string>>("Items", CreateDefaultValueCallback = true)]
public partial class MyControl : UserControl
{
    // The generator will wire up this partial method to instantiate a new object per instance.
    private static partial List<string> GetItemsDefaultValue() => new List<string>();
}

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DPG0005: Invalid Callback Signature (OverrideMetadata)

Non-WPF platforms (such as UWP, WinUI, Uno, and MAUI) do not provide the "old value" when you override property metadata (OverrideMetadata). This error occurs when your callback signature attempts to receive oldValue on a platform that does not support it.

❌ Incorrect code:

// Error: The underlying framework (WinUI/Uno) cannot provide the old value.
[OverrideMetadata<int>("Count", OnChanged = nameof(OnCountChanged))]
public partial class MyWinUIControl : UserControl
{
    private void OnCountChanged(MyWinUIControl sender, int oldValue, int newValue) { }
}

✅ Correct code:

[OverrideMetadata<int>("Count", OnChanged = nameof(OnCountChanged))]
public partial class MyWinUIControl : UserControl
{
    // Solution: Change the signature to only receive the new value.
    private void OnCountChanged(MyWinUIControl sender, int newValue) { }
}

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DPG0007: Unsupported Callback Signature

The generator found a method matching the partial void On{PropertyName}Changed(...) naming convention, but its argument signature is invalid (for example, it uses the generic DependencyObject).

❌ Incorrect code:

[DependencyProperty<string>("Text")]
public partial class MyControl : UserControl
{
    // Error: Using generic DependencyObject and RoutedEventArgs.
    partial void OnTextChanged(DependencyObject d, DependencyPropertyChangedEventArgs e);
}

✅ Correct code:

[DependencyProperty<string>("Text")]
public partial class MyControl : UserControl
{
    // Solution: Use strongly-typed arguments.
    partial void OnTextChanged(string? oldValue, string? newValue);

    // Or include the sender:
    // partial void OnTextChanged(MyControl sender, string? oldValue, string? newValue);
}

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DPG0008: Invalid Default Value Expression

The C# string expression provided in DefaultValueExpression contains syntax errors, causing the Roslyn parser to fail.

❌ Incorrect code:

// Error: Missing closing parenthesis, typo, etc.
[DependencyProperty<string>("Text", DefaultValueExpression = "new(123, ")]
public partial class MyControl : UserControl
{
}

✅ Correct code:

// Solution: Provide a valid C# expression string.
[DependencyProperty<string>("Text", DefaultValueExpression = "new(123, 456)")]
public partial class MyControl : UserControl
{
}

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