ResizableControlExtender

The ResizableControlExtender allows users to resize an element by dragging its edges or a resize handle (typically positioned at the bottom-right corner). It supports optional minimum and maximum size constraints to prevent elements from becoming too small or too large.

Original Ajax Control Toolkit documentation: https://www.asp.net/ajax/ajaxcontroltoolkit/ResizableControlExtender

Features Supported in Blazor

• TargetControlID — ID of the element to make resizable
• HandleCssClass — CSS class for the resize handle element
• ResizableCssClass — CSS class applied while resizing
• MinimumWidth — Minimum width constraint in pixels
• MinimumHeight — Minimum height constraint in pixels
• MaximumWidth — Maximum width constraint in pixels (0 = no limit)
• MaximumHeight — Maximum height constraint in pixels (0 = no limit)
• Enabled — Enable or disable the extender behavior
• BehaviorID — Optional identifier for JavaScript behavior lookup

Web Forms Syntax

<asp:Panel ID="pnlResizable" runat="server" style="width: 300px; height: 200px; border: 1px solid black;">
    <p>Resize me from the bottom-right corner!</p>
    <div id="resizeHandle" class="resize-handle" style="position: absolute; bottom: 0; right: 0; width: 10px; height: 10px; background: #333; cursor: nwse-resize;"></div>
</asp:Panel>

<ajaxToolkit:ResizableControlExtender
    ID="resize1"
    runat="server"
    TargetControlID="pnlResizable"
    HandleCssClass="resize-handle"
    MinimumWidth="100"
    MinimumHeight="100"
    MaximumWidth="600"
    MaximumHeight="500" />

Blazor Migration

<Panel ID="pnlResizable" style="width: 300px; height: 200px; border: 1px solid black;">
    <p>Resize me from the bottom-right corner!</p>
    <div id="resizeHandle" class="resize-handle" style="position: absolute; bottom: 0; right: 0; width: 10px; height: 10px; background: #333; cursor: nwse-resize;"></div>
</Panel>

<ResizableControlExtender
    TargetControlID="pnlResizable"
    HandleCssClass="resize-handle"
    MinimumWidth="100"
    MinimumHeight="100"
    MaximumWidth="600"
    MaximumHeight="500" />

Migration is simple: Remove the ajaxToolkit: prefix and runat="server". Everything else stays the same!

Properties Reference

Property	Type	Default	Description
TargetControlID	string	(required)	ID of the element to make resizable
HandleCssClass	string	""	CSS class for the resize handle element (usually positioned at bottom-right)
ResizableCssClass	string	""	CSS class applied to the target while it is being resized
MinimumWidth	int	0	Minimum width constraint in pixels; 0 = no limit
MinimumHeight	int	0	Minimum height constraint in pixels; 0 = no limit
MaximumWidth	int	0	Maximum width constraint in pixels; 0 = no limit
MaximumHeight	int	0	Maximum height constraint in pixels; 0 = no limit
BehaviorID	string	TargetControlID	Optional identifier for JavaScript behavior lookup
Enabled	bool	true	Whether the extender is active

Usage Examples

Basic Resizable Panel

@rendermode InteractiveServer

<Panel ID="pnlBox" style="width: 300px; height: 200px; border: 1px solid #ccc; padding: 10px; position: relative; overflow: auto;">
    <p>Drag the handle in the corner to resize this panel.</p>
    <div class="resize-handle"></div>
</Panel>

<ResizableControlExtender TargetControlID="pnlBox" HandleCssClass="resize-handle" />

<style>
    .resize-handle {
        position: absolute;
        bottom: 0;
        right: 0;
        width: 15px;
        height: 15px;
        background: #333;
        cursor: nwse-resize;
    }
</style>

Resizable with Size Constraints

@rendermode InteractiveServer

<Panel ID="pnlWindow" style="width: 400px; height: 300px; border: 2px solid #2196F3; position: relative; background: white;">
    <div style="background: #2196F3; color: white; padding: 10px; font-weight: bold;">Resizable Window</div>
    <div style="padding: 20px; overflow: auto; height: calc(100% - 50px);">
        <p>Minimum size: 200x150px</p>
        <p>Maximum size: 800x600px</p>
    </div>
    <div class="resize-corner"></div>
</Panel>

<ResizableControlExtender
    TargetControlID="pnlWindow"
    HandleCssClass="resize-corner"
    MinimumWidth="200"
    MinimumHeight="150"
    MaximumWidth="800"
    MaximumHeight="600"
    ResizableCssClass="resizing" />

<style>
    .resize-corner {
        position: absolute;
        bottom: 0;
        right: 0;
        width: 20px;
        height: 20px;
        background: linear-gradient(135deg, transparent 50%, #2196F3 50%);
        cursor: nwse-resize;
    }
    
    .resizing {
        opacity: 0.8;
    }
</style>

Resizable Text Area

@rendermode InteractiveServer

<Panel ID="pnlTextArea" style="width: 500px; height: 300px; position: relative; border: 1px solid #999;">
    <TextArea ID="txtContent" style="width: 100%; height: 100%; border: none; padding: 10px; resize: none;"></TextArea>
    <div class="resize-handle"></div>
</Panel>

<ResizableControlExtender
    TargetControlID="pnlTextArea"
    HandleCssClass="resize-handle"
    MinimumWidth="250"
    MinimumHeight="150"
    MaximumWidth="1000"
    MaximumHeight="800" />

<style>
    .resize-handle {
        position: absolute;
        bottom: 2px;
        right: 2px;
        width: 18px;
        height: 18px;
        background-image: repeating-linear-gradient(45deg, #ccc, #ccc 2px, transparent 2px, transparent 5px);
        cursor: nwse-resize;
    }
</style>

HTML Output

The ResizableControlExtender produces no HTML itself — it attaches JavaScript behavior to the target element. You must provide the resize handle as part of your panel's HTML.

JavaScript Interop

The ResizableControlExtender loads resizable-control-extender.js as an ES module. JavaScript handles:

• Detecting mouse down on the resize handle
• Tracking mouse movement while resizing
• Updating the element's width and height in real-time
• Enforcing minimum and maximum size constraints
• Applying/removing resize CSS classes
• Handling mouse up to stop resizing
• Preventing text selection during resize

Render Mode Requirements

The ResizableControlExtender requires InteractiveServer render mode:

@rendermode InteractiveServer

Graceful Degradation

• SSR/Static mode: The extender silently skips initialization. The Panel displays at its initial size.
• JavaScript disabled: Same as SSR — Panel is not resizable.

Migration Notes

From Web Forms Ajax Toolkit

1. Remove ajaxToolkit: prefix

   - <ajaxToolkit:ResizableControlExtender
   + <ResizableControlExtender
   

2. Remove runat="server" and ID attributes

3. Size constraints use integers, not strings

   - MinimumWidth="200"
   + MinimumWidth="200"
   

Before (Web Forms)

<asp:Panel ID="pnlBox" runat="server" style="width: 300px; height: 200px; border: 1px solid black; position: relative;">
    <p>Content</p>
    <div class="handle"></div>
</asp:Panel>

<ajaxToolkit:ResizableControlExtender
    ID="resize1"
    TargetControlID="pnlBox"
    HandleCssClass="handle"
    MinimumWidth="150"
    MaximumWidth="800"
    runat="server" />

After (Blazor)

<Panel ID="pnlBox" style="width: 300px; height: 200px; border: 1px solid black; position: relative;">
    <p>Content</p>
    <div class="handle"></div>
</Panel>

<ResizableControlExtender
    TargetControlID="pnlBox"
    HandleCssClass="handle"
    MinimumWidth="150"
    MaximumWidth="800" />

Best Practices

1. Use position: relative or absolute — Ensure the target has proper positioning for the handle
2. Style the handle clearly — Make the resize handle visually obvious with contrasting colors
3. Set overflow: auto — Use overflow: auto on the panel so content scrolls when resized small
4. Provide constraints — Set reasonable min/max sizes to prevent broken layouts
5. Test edge cases — Verify behavior when resizing to minimum/maximum sizes

Troubleshooting

Issue	Solution
Handle not visible	Verify HandleCssClass references a styled element within the panel. Check CSS positioning.
Resizing not working	Ensure @rendermode InteractiveServer is set and TargetControlID matches the panel's ID.
Size constraints ignored	Verify min/max values are integers and make sense (min < max). A value of 0 means no limit.
Content overlaps handle	Adjust panel padding or use overflow: auto to prevent content overlap.

See Also

• Ajax Control Toolkit Overview — How extenders work and render mode requirements
• Panel Component — The Panel control (documentation coming soon)
• DragPanelExtender — Make panels draggable
• Original Ajax Control Toolkit: https://www.asp.net/ajax/ajaxcontroltoolkit