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Inline C# in XAML

Overview

AXSG supports valid-XAML inline C# through an explicit CSharp surface in the default Avalonia XML namespace. This feature is intended for cases where compact binding or event syntax is no longer expressive enough and the author needs normal C# expressions or event statements with access to the local XAML context.

The feature is deliberately explicit:

• The XAML stays valid XML and valid XAML.
• Inline code is opt-in through CSharp.
• Existing shorthand expression forms remain supported, but CSharp is the canonical form for multi-line or context-rich code.

Namespace

CSharp is available directly in the default Avalonia XML namespace. No additional xmlns prefix is required.

Supported forms

Compact attribute form

Use this form for short expressions or short event lambdas.

<TextBlock Text="{CSharp Code=source.ProductName}" />
<Button Click="{CSharp Code='(sender, e) => source.ClickCount++'}" />

Notes:

• The compact form is best for short single-line code.
• Complex expressions with commas, nested braces, or long interpolated strings should use the object-element form below.

Object-element form

Use this form for multi-line expressions or event statement blocks.

<TextBlock.Text>
  <CSharp><![CDATA[
source.ProductName + " #" + source.ClickCount
  ]]></CSharp>
</TextBlock.Text>

<Button.Click>
  <CSharp><![CDATA[
source.ClickCount++;
source.LastAction = "inline code";
  ]]></CSharp>
</Button.Click>

Context variables

Inline C# is compiled against explicit context variables.

Value code

Value code is compiled as a C# expression and can access:

• source: the ambient x:DataType instance when available, otherwise object
• root: the root x:Class instance when available, otherwise object
• target: the current target object for the assignment

Example:

<TextBlock.Text>
  <CSharp><![CDATA[
$"{source.Quantity}x {source.ProductName}"
  ]]></CSharp>
</TextBlock.Text>

Event code

Event code can be either:

• a lambda expression, or
• a statement block body

Event code can access:

• source
• root
• target
• sender
• e
• arg0, arg1, ... for delegate parameters beyond the common two-parameter form

Example:

<Button.Click>
  <CSharp><![CDATA[
source.ClickCount++;
source.LastAction = $"Clicked by {sender}";
  ]]></CSharp>
</Button.Click>

Semantics

Value positions

• Value code must evaluate to a value compatible with the target property.
• AXSG emits a generated binding expression and reevaluates it when tracked source members change.
• Dependency tracking is based on instance property and field reads from source.
• root and target are available to the code, but they are treated as contextual values, not dependency-tracked binding inputs.

Event positions

• Event code is compiled into a generated wrapper method.
• For lambda form, the lambda is compiled against the event delegate type.
• For statement-block form, AXSG generates a method body with the context variables in scope.
• Async lambdas or async statement handlers are not supported in this feature.

Valid placement

Supported placements:

• property attributes through the compact form
• property elements through the object-element form
• event attributes through the compact form
• event property elements through the object-element form
• style and control-theme setter values through property elements or compact form

Unsupported placements:

• inline code as a standalone object in the visual tree
• inline code inside selector strings
• inline code inside resource keys

Language service behavior

The language service treats CSharp content as real C#.

Supported tooling:

• completion
• hover
• semantic colorization
• go to definition
• find references
• inlay type hints where applicable

For object-element code blocks, the language service resolves the local XAML context first and then analyzes the block against the generated C# context.

Diagnostics

Representative diagnostics:

• invalid value expression for the target property
• event code that does not match the event delegate shape
• async event code not supported
• inline code block used outside a value or event property context
• inline code that depends on unresolved source, root, or target members

Design constraints

• The feature must not require invalid XAML syntax.
• Runtime fallback paths may skip direct runtime reload when inline C# is present; generated reload remains the supported path.
• The compiler, runtime, and language service must share the same context model so navigation and generated code stay consistent.