Composite controls

Composite controls are a new way of declaring controls added in version 4.0. The main use case for composite controls is to offer an easy method of combining existing controls into larger blocks, and being able to parameterize these blocks.

Declare the composite control

The composite control must satisfy the following two requirements:

  • Inherit from the CompositeControl class.
  • Contain the static or instance method called GetContents. This method can have any parameters (they will be interpreted as control properties), and must return either DotvvmControl or IEnumerable<DotvvmControl>.

The declaration can look like this:

public class ImageButton : CompositeControl
    public static DotvvmControl GetContents(
        [DefaultValue("Button")] ValueOrBinding<string> text,
        ICommandBinding click,
        string imageUrl = "/icons/default-image.png"
        return new LinkButton()
            .SetProperty(b => b.Click, click)
                new HtmlGenericControl("img")
                    .SetAttribute("src", imageUrl),
                new Literal(text)

Composite control properties

The code snippet above declared the ImageButton control with 3 properties:

  • Text - the property is of type ValueOrBinding<string>. You can assign both value binding or a hard-coded value to the control. Also, the property has a default value - if it is not specified, the text "Button" will be used.

  • Click - the property specifies ICommandBinding as a type, so only the command or staticCommand binding can be assigned to the property. It doesn't have a default value, therefore it is required.

  • ImageUrl - the property is declared as string, so it will support only hard-coded values in markup (or a resource binding). It has a default value.

You can also use the following types of parameters:

  • ITemplate for passing templates - you can then use the TemplateHost to instantiate the template in the generated controls.

  • DotvvmControl or IEnumerable<DotvvmControl> for passing child elements - you can use for example the AppendChildren method to place the controls as children in the generated controls.

  • IValueBinding<T> for properties that allow only value binding expressions.

You can use nullable types - in general, they tell DotVVM that the property is optional. You can specify the default value either as a default value of the parameter, or via the [DefaultValue] attribute. If the property is not nullable and doesn't provide a default value, it will be treated as a required property.

You can use the MarkupOptions attributes or the DataContextChange attributes on the properties, same as on the properties in markup or code-only controls.

If the property is named Content or ContentTemplate, it will be the default content property - any controls inside of the CompositeControl will be placed into this property.

See the Control properties for more information about declaring control properties.

If a parameter of type IDotvvmRequestContext is added to GetContents, it will not create a new property and instead simply pass the current request context into the method.

Fluent API for building control hierarchies

In order to simplify building control hierarchy, DotVVM adds several extension methods. The most important are:

  • SetProperty - can set a property to the control.

    • It supports specifying the property via a lambda expression or by passing the DotvvmProperty object
    • It supports specifying the property as its name, which allows setting properties of other composite controls or markup controls using the @property directive.
    • It supports the ValueOrBinding<T> types, it will internally call either SetValue or SetBinding.
  • SetAttribute - sets a HTML attribute. It is commonly used for HtmlGenericControl.

  • SetCapability - sets a control capability property.

  • AddCssClass - adds a CSS class. It can be called multiple times.

    • AddCssClass(className, condition) overload can be used to add a conditional CSS class. The condition may be a value binding.
  • AddCssStyle - add an inline CSS style into the style attribute. The style value may be a binding.

  • AppendChildren - appends children to the control's children collection.

Using Markup controls in composite controls

If you plan to instantiate a markup control as a child inside a composite control, you should use the MarkupControlContainer class. See the Markup controls page for more info.


PostBack.Handlers is a collection of postback handler which can be added onto any control. The postback handlers are automatically applied to postback expressions the control creates using the KnockoutHelper.GenerateClientPostBackExpression method. However, composite controls don't generate any postback expressions and delegate this work to child components, so the Postback.Handlers collection must be copied to the same child components where command bindings are passed.

As of version 4.2, PostBack.Handlers set on the composite control are copied into the child components automatically. The cloning procedure takes place after the GetContents method is invoked, so any controls created in the method should have the correct postback handlers applied.

Explicit action might be needed if additional controls are generated outside the GetContents, for example inside a DelegateTemplate. In such case, apply the this.CopyPostBackHandlersRecursive(control) on the control which might need a postback handler from the parent composite.

public DotvvmControl GetContents(
    IValueBinding<IEnumerable<string>> dataSource,
    ICommandBinding itemClick = null,
    ICommandBinding headerClick = null
    var repeater = new Repeater() {
        WrapperTagName = "div",
        ItemTemplate = new DelegateTemplate(() =>
            // postback handlers must be explicitly cloned
            this.CopyPostBackHandlersRecursive(new Button("Item", itemClick))
    .SetProperty(Repeater.DataSourceProperty, dataSource);
    return new HtmlGenericControl("div")
            new HtmlGenericControl("h3") { InnerText = "List of buttons" }
                // no action needed, this command will be found automatically
                .SetBinding(Events.Click, headerClick),

In the example above, we create a list of buttons with one clickable title above. The buttons are created inside DelegateTemplate, so we must copy the postback handler explicitly.

Note that in this simple case, it would be easier to use new CloneTemplate(new Button("Item", itemClick)) instead. In that case, DotVVM is able to set the postback handlers automatically.

The postback handlers are only applied onto controls which have the same command as the composite control. No postback handlers will be automatically applied if a new command binding is created (even when derived from an existing command). You can always override this behavior by explicitly copying the matching Postback.Handlers collection.


By default, the GetContents runs for each HTTP request, potentially many times if the control is in a Repeater. Instead, it is possible to evaluate the control only once, during the dothtml compilation using the [ControlMarkupOptions(Precompile = ControlPrecompilationMode.Always)] attribute on the component. The available precompilation modes are:

  • Never - Never attempt precompilation (the default).
  • IfPossibleAndIgnoreExceptions - Attempt precompilation whenever it's possible. If exception is thrown by the control, it is ignored and precompilation is skipped.
  • IfPossible - Attempt precompilation whenever it's possible. If exception is thrown by the control, compilation fails.
  • Always - Always try to precompile the control and fail compilation when it's not possible.
  • InServerSideStyles - Always precompile this controls and do that while styles are being processed. This will allow other styles to process the generated controls AutoUI.

The precompilation is deemed impossible if a resource binding is used in a property which does not support ValueOrBinding. The control may also throw the SkipPrecompilationException to suppress precompilation in certain cases.

Precompilation is especially valuable if the composite control is expensive to create - if it needs to compile new bindings or scan objects using reflection, for example. Instead of caching the individual bindings, the entire generated control tree can be cached.

In the following example, we create a simplistic form control which simply creates a textbox for each property of the current data context.

// "Always" Precompile - the CreateBinding is slow and we want to avoid performance surprises.
// In case the precompilation isn't possible, we will get a compilation error
[ControlMarkupOptions(Precompile = ControlPrecompilationMode.Always)]
public class ObjectEditor
    public ObjectEditor(BindingCompilationService bindingService)
        this.bindingService = bindingService;
    public IEnumerable<DotvvmControl> GetContents()
        foreach (var property of this.GetDataContextType().DataContextType.GetProperties())
            var binding = CreateBinding(property);

            yield return new HtmlGenericControl("div")
                    new Literal($"{property.Name}: "),
                    new TextBox().SetProperty(t => t.Text, binding)

    IValueBinding CreateBinding(PropertyInfo property)
        DataContextStack dataContext = this.GetDataContextType();
        // DotVVM bindings are built using System.Linq.Expressions
        // _this is represented as a parameter expression with BindingParameterAnnotation of the current data context
        Expression _this =
            Expression.Parameter(dataContext.DataContextType, "_this")
                      .AddParameterAnnotation(new BindingParameterAnnotation(dataContext));
        Expression expression = Expression.Property(_this, property);

        // To initialize the binding, we have to specify the current data context and the expression.
        // The constructor automatically creates the server-side compiled delegate and the Knockout.js binding.
        // This takes rather long time to compile, which is why we enforce 
        return new ValueBindingExpression(this.bindingService, new object[] {
            new ParsedExpressionBindingProperty(expression),

There is number of limitations on the precompiled controls:

  • Resource bindings can only be used in properties of type ValueOrBinding<T>
  • IDotvvmRequestContext isn't available
  • this.Parent isn't available
  • All child controls are of type ResolvedControlHelper.LazyRuntimeControl. In you need access to the the real control, call the the GetLogicalChildren() method on it.
  • this.GetDotvvmUniqueId and this.CreateClientId methods currently don't work
  • OnInit, OnLoad, OnPreRender and the Render methods are ignored
  • At runtime, the control is replaced by PrecompiledControlPlaceholder which only contains its content.

See also