What causes "Operation cannot be performed." in dotnet/wpf?

Truncate the string to the control's limit before calling SetValue.

dotnet/wpf · error · InvalidOperationException

Operation cannot be performed.

Error message

Operation cannot be performed.

What it means

The Win32 edit box proxy throws InvalidOperationException(OperationCannotBePerformed) from IValueProvider.SetValue when the requested text is longer than the control's text limit. It queries EM_GETLIMITTEXT; if a limit is set (result != -1) and the string exceeds it, the operation is rejected before sending WM_SETTEXT.

Solutions

Truncate the string to the control's limit before calling SetValue.
Raise the limit in the target application via EM_SETLIMITTEXT.
Use a multiline/large control (or write to the data source directly) for long content.
Catch InvalidOperationException and split input into allowed-size chunks if the control semantics permit.

Example fix

// before
valuePattern.SetValue(longText); // throws if longText.Length > limit
// after
int limit = /* EM_GETLIMITTEXT result, or int.MaxValue if -1 */;
valuePattern.SetValue(longText.Length > limit ? longText.Substring(0, limit) : longText);

Defensive patterns

Strategy: validation

Validate before calling

int limit = SendMessage(hwnd, EM_GETLIMITTEXT, 0, 0);
if (limit != -1 && text.Length > limit)
    text = text.Substring(0, limit); // or reject

Try / catch

try { vp.SetValue(text); }
catch (InvalidOperationException) { /* text exceeds EM limit; truncate or split */ }

Prevention

Query EM_GETLIMITTEXT before writing.
Keep generated test data within control limits.
Raise EM_SETLIMITTEXT in the app when longer content is required.

When it happens

Trigger: Calling IValueProvider.SetValue with a string whose length exceeds the value returned by EM_GETLIMITTEXT for the edit control (limit set via EM_SETLIMITTEXT or the control default, e.g. 32766 chars).

Common situations: Pasting large log text or multiline blobs into a short input field; a test framework filling a max-length-limited field with generated data; migrating scripts between controls with different limits.

Understand the failure class

Background: "value must be between 0 and 1" / "out of range" / "must not be negative" errors: fixing range-validation failures across open-source libraries — this error's family across 42 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/26118d988ec5ee35. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/UIAutomation/UIAutomationClientSideProviders/MS/Internal/AutomationProxies/WindowsEditBox.cs:293

            if (Misc.IsBitSet(styles, NativeMethods.ES_NUMBER))
            {
                // check if string contains any non-numeric characters.
                foreach (char ch in str)
                {
                    if (char.IsLetter (ch))
                    {
                        throw new ArgumentException(SR.Format(SR.NotAValidValue, str), "val");
                    }
                }
            }

            // Text/edit box should not enter more characters than what is allowed through keyboard.
            // Determine the max number of chars this editbox accepts
            int result = Misc.ProxySendMessageInt(_hwnd, NativeMethods.EM_GETLIMITTEXT, IntPtr.Zero, IntPtr.Zero);
            // A result of -1 means that no limit is set.
            if (result != -1 && result < str.Length)
            {
                throw new InvalidOperationException (SR.OperationCannotBePerformed);
            }

            // Send the message...
            result = Misc.ProxySendMessageInt(_hwnd, NativeMethods.WM_SETTEXT, IntPtr.Zero, new StringBuilder(str));
            if (result != 1)
            {
                throw new InvalidOperationException(SR.OperationCannotBePerformed);
            }
        }

        // Request to get the value that this UI element is representing as a string
        string IValueProvider.Value
        {
            get
            {
                return GetValue();
            }
        }

View on GitHub (pinned to 81131a70a4)