dotnet/wpf · error · InvalidOperationException

SR.Format(SR.PropertyIsInitializeOnly, "DataType"…

Error message

SR.Format(SR.PropertyIsInitializeOnly, "DataType", this.GetType().Name)

What it means

The DataType property of TemplateKey is write-once and only during the initialization window. If a setter is invoked outside of the BeginInit/EndInit pair, or DataType is set to a different value after it was already assigned, the property setter throws InvalidOperationException. This guarantees template keys are immutable once committed.

Solutions

  1. Call BeginInit() before assigning DataType and EndInit() afterwards.
  2. Create a new TemplateKey instance instead of mutating DataType on an existing one.
  3. If a different type is needed, construct a fresh key with the desired DataType in the constructor-supported flow.

Example fix

// before
var key = new TemplateKey();
key.DataType = typeof(Customer); // throws: not initializing
// after
var key = new TemplateKey();
key.BeginInit();
key.DataType = typeof(Customer);
key.EndInit();
Defensive patterns

Strategy: validation

Validate before calling

if (key.DataType != null)
    throw new InvalidOperationException("TemplateKey.DataType already set; create a new key.");

Try / catch

try { key.DataType = value; }
catch (InvalidOperationException) { key = CreateNewKey(value); }

Prevention

When it happens

Trigger: Setting TemplateKey.DataType directly without calling BeginInit() first; setting DataType again (to a different value) after it was already assigned even inside an initialization block.

Common situations: Developer treats DataType like a normal mutable property; tooling that reuses and reconfigures a TemplateKey instance for a second template; data-binding attempts that try to assign DataType after creation.

Understand the failure class

Background: "Invalid state transition" errors: "status must be X, actually Y", "already rejected/charging/uninstalled", "cannot ... while running" — what they mean when a library rejects your call — this error's family across 31 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/8ee3141ede0c73a6. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/TemplateKey.cs:69

            }

            _initializing = false;
        }

#endregion ISupportInitialize

        /// <summary>
        /// The type for which the template is designed.  This is either
        /// a Type (for object data), or a string (for XML data).  In the latter
        /// case the string denotes the XML tag name.
        /// </summary>
        public object DataType
        {
            get { return _dataType; }
            set
            {
                if (!_initializing)
                    throw new InvalidOperationException(SR.Format(SR.PropertyIsInitializeOnly, "DataType", this.GetType().Name));
                if (_dataType != null && value != _dataType)
                    throw new InvalidOperationException(SR.Format(SR.PropertyIsImmutable, "DataType", this.GetType().Name));

                Exception ex = ValidateDataType(value, "value");
                if (ex != null)
                    throw ex;

                _dataType = value;
            }
        }

        /// <summary> Override of Object.GetHashCode() </summary>
        public override int GetHashCode()
        {
            // note that the hash code can change, but only during intialization
            // and only once (DataType can only be changed once, from null to
            // non-null, and that can only happen during [Begin/End]Init).
            // Technically this is still a violation of the "constant during

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