MaskedEditExtender

March 15, 2026 · View on GitHub

The MaskedEditExtender applies an input mask to a target TextBox, restricting and formatting user input according to a mask pattern. It supports number, date, time, and custom masks with configurable prompt characters, input direction, and validation behavior.

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

Features Supported in Blazor

TargetControlID — ID of the TextBox to apply the mask to
Mask — The mask pattern string
MaskType — The type of mask (None, Number, Date, Time, DateTime)
InputDirection — Direction of text input (LeftToRight, RightToLeft)
PromptCharacter — Character displayed for unfilled mask positions
AutoComplete — Whether to enable browser autocomplete
AutoCompleteValue — Value to use for autocomplete
Filtered — Additional characters allowed beyond the mask definition
ClearMaskOnLostFocus — Remove mask characters when input loses focus
ClearTextOnInvalid — Clear text when it doesn't match the mask on blur
AcceptAMPM — Accept AM/PM designator for time masks
AcceptNegative — How negative values are displayed for number masks
ErrorTooltipEnabled — Show tooltip on validation failure
ErrorTooltipCssClass — CSS class for the error tooltip
Enabled — Enable or disable the extender behavior
BehaviorID — Optional identifier for JavaScript behavior lookup

Mask Characters

Character	Description
`9`	Digit (0–9)
`L`	Letter (a–z, A–Z)
`$`	Digit or space
`C`	Any character
`A`	Letter or digit
`N`	Digit or letter (same as A)
`?`	Any character (optional)

MaskType Enum

enum MaskType
{
    None = 0,      // Custom mask (default)
    Number = 1,    // Numeric mask
    Date = 2,      // Date mask
    Time = 3,      // Time mask
    DateTime = 4   // Date and time mask
}

InputDirection Enum

enum InputDirection
{
    LeftToRight = 0,  // Standard left-to-right input (default)
    RightToLeft = 1   // Right-to-left input (for currency, etc.)
}

AcceptNegative Enum

enum AcceptNegative
{
    None = 0,   // No negative values allowed (default)
    Left = 1,   // Negative sign on the left (e.g., -123)
    Right = 2   // Negative sign on the right (e.g., 123-)
}

Web Forms Syntax

<asp:TextBox ID="txtPhone" runat="server" />

<ajaxToolkit:MaskedEditExtender
    ID="mee1"
    runat="server"
    TargetControlID="txtPhone"
    Mask="(999) 999-9999"
    MaskType="None"
    InputDirection="LeftToRight"
    PromptCharacter="_"
    ClearMaskOnLostFocus="true" />

Blazor Migration

<TextBox ID="txtPhone" />

<MaskedEditExtender
    TargetControlID="txtPhone"
    Mask="(999) 999-9999"
    MaskType="MaskType.None"
    InputDirection="InputDirection.LeftToRight"
    PromptCharacter="_"
    ClearMaskOnLostFocus="true" />

Migration is simple: Remove the ajaxToolkit: prefix and runat="server". Use full enum type names for MaskType and InputDirection. Everything else stays the same!

Properties Reference

Property	Type	Default	Description
`TargetControlID`	`string`	(required)	ID of the TextBox to apply the mask to
`Mask`	`string`	`""`	Mask pattern string (e.g., "999-999-9999" for phone)
`MaskType`	`MaskType`	`None`	Type of mask: `None`, `Number`, `Date`, `Time`, `DateTime`
`InputDirection`	`InputDirection`	`LeftToRight`	Direction of text input: `LeftToRight` or `RightToLeft`
`PromptCharacter`	`string`	`"_"`	Character displayed for unfilled mask positions
`AutoComplete`	`bool`	`false`	Whether to enable browser autocomplete on the target input
`AutoCompleteValue`	`string`	`""`	Value to use for autocomplete when enabled
`Filtered`	`string`	`""`	Additional characters allowed beyond the mask definition
`ClearMaskOnLostFocus`	`bool`	`true`	Remove mask characters when input loses focus
`ClearTextOnInvalid`	`bool`	`false`	Clear text when it doesn't match the mask on blur
`AcceptAMPM`	`bool`	`false`	Accept AM/PM designator for time masks
`AcceptNegative`	`AcceptNegative`	`None`	How negative values are displayed: `None`, `Left`, `Right`
`ErrorTooltipEnabled`	`bool`	`false`	Show tooltip with error info on validation failure
`ErrorTooltipCssClass`	`string`	`""`	CSS class applied to the error tooltip
`BehaviorID`	`string`	TargetControlID	Optional identifier for JavaScript behavior lookup
`Enabled`	`bool`	`true`	Whether the extender is active

Usage Examples

Phone Number Mask

@rendermode InteractiveServer

<TextBox ID="txtPhone" />

<MaskedEditExtender
    TargetControlID="txtPhone"
    Mask="(999) 999-9999"
    ClearMaskOnLostFocus="true" />

Date Mask

@rendermode InteractiveServer

<TextBox ID="txtDate" />

<MaskedEditExtender
    TargetControlID="txtDate"
    Mask="99/99/9999"
    MaskType="MaskType.Date"
    ClearMaskOnLostFocus="false" />

Currency Mask with Negative Support

@rendermode InteractiveServer

<TextBox ID="txtAmount" />

<MaskedEditExtender
    TargetControlID="txtAmount"
    Mask="999,999.99"
    MaskType="MaskType.Number"
    InputDirection="InputDirection.RightToLeft"
    AcceptNegative="AcceptNegative.Left" />

Time Mask with AM/PM

@rendermode InteractiveServer

<TextBox ID="txtTime" />

<MaskedEditExtender
    TargetControlID="txtTime"
    Mask="99:99"
    MaskType="MaskType.Time"
    AcceptAMPM="true" />

@rendermode InteractiveServer

<TextBox ID="txtSSN" />

<MaskedEditExtender
    TargetControlID="txtSSN"
    Mask="999-99-9999"
    PromptCharacter="#"
    ClearMaskOnLostFocus="true"
    ClearTextOnInvalid="true"
    ErrorTooltipEnabled="true" />

HTML Output

The MaskedEditExtender produces no HTML itself — it attaches JavaScript behavior to the target TextBox that intercepts keystrokes and enforces the mask pattern.

JavaScript Interop

The MaskedEditExtender loads masked-edit-extender.js as an ES module. JavaScript handles:

Intercepting keystrokes and enforcing mask patterns
Positioning the cursor within the mask
Handling paste operations (stripping invalid characters)
Displaying prompt characters for unfilled positions
Clearing/showing the mask on focus/blur
Error tooltip display on validation failure

Render Mode Requirements

The MaskedEditExtender requires InteractiveServer render mode:

@rendermode InteractiveServer

Graceful Degradation

SSR/Static mode: The extender silently skips initialization. The TextBox works as a plain text input without masking.
JavaScript disabled: Same as SSR — TextBox functions without mask enforcement.

Migration Notes

From Web Forms Ajax Toolkit

Remove ajaxToolkit: prefix

- <ajaxToolkit:MaskedEditExtender
+ <MaskedEditExtender

Remove runat="server" and ID attributes

Use full enum type names

- MaskType="Number"
+ MaskType="MaskType.Number"
- InputDirection="RightToLeft"
+ InputDirection="InputDirection.RightToLeft"

Before (Web Forms)

<asp:TextBox ID="txtPhone" runat="server" />

<ajaxToolkit:MaskedEditExtender
    ID="mee1"
    TargetControlID="txtPhone"
    Mask="(999) 999-9999"
    ClearMaskOnLostFocus="true"
    runat="server" />

After (Blazor)

<TextBox ID="txtPhone" />

<MaskedEditExtender
    TargetControlID="txtPhone"
    Mask="(999) 999-9999"
    ClearMaskOnLostFocus="true" />

Best Practices

Choose the right MaskType — Use Number, Date, or Time for built-in validation
Set ClearMaskOnLostFocus wisely — true shows clean values; false shows the mask structure
Use ClearTextOnInvalid — Prevents partial entries from being submitted
Pair with validators — Combine with RequiredFieldValidator or CustomValidator for server-side validation
Test with paste — Ensure pasted values are properly filtered

Troubleshooting

Issue	Solution
Mask not appearing	Verify `TargetControlID` matches the TextBox's `ID`. Ensure `@rendermode InteractiveServer` is set.
Wrong characters accepted	Check `Mask` pattern uses correct mask characters (9, L, $, C, A).
Cursor jumping unexpectedly	Verify `InputDirection` matches your intended input flow.
Negative values not accepted	Set `AcceptNegative` to `Left` or `Right` and ensure `MaskType` is `Number`.
Error tooltip not showing	Set `ErrorTooltipEnabled="true"` and optionally provide `ErrorTooltipCssClass`.