reactiveui · ChrisPulman · Aug 17, 2026 · Aug 16, 2026 · Aug 16, 2026
diff --git a/README.md b/README.md
@@ -435,6 +435,9 @@ public partial class MyReactiveClass : ReactiveObject
 }
 ```
 
+String values are automatically quoted, so `InitialValue = ""` produces an empty string initial value.
+Non nullable `string` properties are initialized with `string.Empty` when no `InitialValue` is provided.
+
 ## Usage ReactiveCommand `[ReactiveCommand]`
 
 ### Usage ReactiveCommand without parameter

diff --git a/...tringInitialValue#ReactiveUI.SourceGenerators.ObservableAsPropertyAttribute.g.verified.cs b/...tringInitialValue#ReactiveUI.SourceGenerators.ObservableAsPropertyAttribute.g.verified.cs
@@ -0,0 +1,56 @@
+//HintName: ReactiveUI.SourceGenerators.ObservableAsPropertyAttribute.g.cs
+// Copyright (c) 2026 .NET Foundation and Contributors. All rights reserved.
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for full license information.
+
+// <auto-generated/>
+#pragma warning disable
+#nullable enable
+namespace ReactiveUI.SourceGenerators;
+
+/// <summary>
+/// ObservableAsPropertyAttribute.
+/// </summary>
+/// <seealso cref="Attribute" />
+[global::System.AttributeUsage(global::System.AttributeTargets.Field | global::System.AttributeTargets.Property | global::System.AttributeTargets.Method, AllowMultiple = false, Inherited = false)]
+internal sealed class ObservableAsPropertyAttribute : global::System.Attribute
+{
+    /// <summary>
+    /// Gets the name of the property.
+    /// </summary>
+    /// <value>
+    /// The name of the property.
+    /// </value>
+    public string? PropertyName { get; init; }
+
+    /// <summary>
+    /// Gets the Readonly state of the OAPH property.
+    /// </summary>
+    /// <value>
+    /// The is read only of the OAPH property.
+    /// </value>
+    public bool ReadOnly { get; init; } = true;
+
+    /// <summary>
+    /// Gets the AccessModifier of the OAPH property.
+    /// </summary>
+    /// <value>
+    /// The AccessModifier of the OAPH property, protected if true.
+    /// </value>
+    public bool UseProtected { get; init; } = false;
+
+    /// <summary>
+    /// Gets the inheritance modifier of the generated property.
+    /// </summary>
+    public InheritanceModifier Inheritance { get; init; }
+
+    /// <summary>
+    /// Gets the Initial Value of the OAPH property. 
+    /// This should be a string representation of the value. i.e. "42" or "DateTime.Now" or "new List&lt;string&gt;()".
+    /// String values are automatically quoted.
+    /// </summary>
+    public string? InitialValue { get; init; }
+}
+#nullable restore
+#pragma warning restore
diff --git a/...ithEmptyStringInitialValue#TestNs.TestVM.ObservableAsPropertyFromObservable.g.verified.cs b/...ithEmptyStringInitialValue#TestNs.TestVM.ObservableAsPropertyFromObservable.g.verified.cs
@@ -0,0 +1,31 @@
+//HintName: TestNs.TestVM.ObservableAsPropertyFromObservable.g.cs
+// <auto-generated/>
+using ReactiveUI;
+
+#pragma warning disable
+#nullable enable
+
+namespace TestNs
+{
+
+    public partial class TestVM
+    {
+        /// <inheritdoc cref="PLCActive"/>
+        private string _pLCActive = "";
+
+        /// <inheritdoc cref="_pLCActiveHelper"/>
+        private readonly ReactiveUI.ObservableAsPropertyHelper<string> _pLCActiveHelper;
+
+        /// <inheritdoc cref="_pLCActive"/>
+        [global::System.Diagnostics.CodeAnalysis.ExcludeFromCodeCoverage]
+        public partial string PLCActive { get => _pLCActive = (_pLCActiveHelper == null ? _pLCActive : _pLCActiveHelper.Value); }
+
+        [global::System.Diagnostics.CodeAnalysis.ExcludeFromCodeCoverage]
+        protected void InitializeOAPH()
+        {
+
+        }
+    }
+}
+#nullable restore
+#pragma warning restore
diff --git a/...thoutInitialValue#ReactiveUI.SourceGenerators.ObservableAsPropertyAttribute.g.verified.cs b/...thoutInitialValue#ReactiveUI.SourceGenerators.ObservableAsPropertyAttribute.g.verified.cs
@@ -0,0 +1,56 @@
+//HintName: ReactiveUI.SourceGenerators.ObservableAsPropertyAttribute.g.cs
+// Copyright (c) 2026 .NET Foundation and Contributors. All rights reserved.
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for full license information.
+
+// <auto-generated/>
+#pragma warning disable
+#nullable enable
+namespace ReactiveUI.SourceGenerators;
+
+/// <summary>
+/// ObservableAsPropertyAttribute.
+/// </summary>
+/// <seealso cref="Attribute" />
+[global::System.AttributeUsage(global::System.AttributeTargets.Field | global::System.AttributeTargets.Property | global::System.AttributeTargets.Method, AllowMultiple = false, Inherited = false)]
+internal sealed class ObservableAsPropertyAttribute : global::System.Attribute
+{
+    /// <summary>
+    /// Gets the name of the property.
+    /// </summary>
+    /// <value>
+    /// The name of the property.
+    /// </value>
+    public string? PropertyName { get; init; }
+
+    /// <summary>
+    /// Gets the Readonly state of the OAPH property.
+    /// </summary>
+    /// <value>
+    /// The is read only of the OAPH property.
+    /// </value>
+    public bool ReadOnly { get; init; } = true;
+
+    /// <summary>
+    /// Gets the AccessModifier of the OAPH property.
+    /// </summary>
+    /// <value>
+    /// The AccessModifier of the OAPH property, protected if true.
+    /// </value>
+    public bool UseProtected { get; init; } = false;
+
+    /// <summary>
+    /// Gets the inheritance modifier of the generated property.
+    /// </summary>
+    public InheritanceModifier Inheritance { get; init; }
+
+    /// <summary>
+    /// Gets the Initial Value of the OAPH property. 
+    /// This should be a string representation of the value. i.e. "42" or "DateTime.Now" or "new List&lt;string&gt;()".
+    /// String values are automatically quoted.
+    /// </summary>
+    public string? InitialValue { get; init; }
+}
+#nullable restore
+#pragma warning restore
diff --git a/...ropertyWithoutInitialValue#TestNs.TestVM.ObservableAsPropertyFromObservable.g.verified.cs b/...ropertyWithoutInitialValue#TestNs.TestVM.ObservableAsPropertyFromObservable.g.verified.cs
@@ -0,0 +1,40 @@
+//HintName: TestNs.TestVM.ObservableAsPropertyFromObservable.g.cs
+// <auto-generated/>
+using ReactiveUI;
+
+#pragma warning disable
+#nullable enable
+
+namespace TestNs
+{
+
+    public partial class TestVM
+    {
+        /// <inheritdoc cref="PLCActive"/>
+        private string _pLCActive = string.Empty;
+
+        /// <inheritdoc cref="_pLCActiveHelper"/>
+        private readonly ReactiveUI.ObservableAsPropertyHelper<string> _pLCActiveHelper;
+
+        /// <inheritdoc cref="_pLCActive"/>
+        [global::System.Diagnostics.CodeAnalysis.ExcludeFromCodeCoverage]
+        public partial string PLCActive { get => _pLCActive = (_pLCActiveHelper == null ? _pLCActive : _pLCActiveHelper.Value); }
+/// <inheritdoc cref="PLCName"/>
+        private string? _pLCName;
+
+        /// <inheritdoc cref="_pLCNameHelper"/>
+        private readonly ReactiveUI.ObservableAsPropertyHelper<string?> _pLCNameHelper;
+
+        /// <inheritdoc cref="_pLCName"/>
+        [global::System.Diagnostics.CodeAnalysis.ExcludeFromCodeCoverage]
+        public partial string? PLCName { get => _pLCName = (_pLCNameHelper == null ? _pLCName : _pLCNameHelper.Value); }
+
+        [global::System.Diagnostics.CodeAnalysis.ExcludeFromCodeCoverage]
+        protected void InitializeOAPH()
+        {
+
+        }
+    }
+}
+#nullable restore
+#pragma warning restore
diff --git a/...InDeeplyNestedClass#TestNs.Level1+Level2.ObservableAsPropertyFromObservable.g.verified.cs b/...InDeeplyNestedClass#TestNs.Level1+Level2.ObservableAsPropertyFromObservable.g.verified.cs
@@ -13,7 +13,7 @@ public partial class Level1
     public partial class Level2
     {
         /// <inheritdoc cref="Level2ObservableProperty"/>
-        private string _level2ObservableProperty;
+        private string _level2ObservableProperty = string.Empty;
 
         /// <inheritdoc cref="_level2ObservablePropertyHelper"/>
         private ReactiveUI.ObservableAsPropertyHelper<string>? _level2ObservablePropertyHelper;

diff --git a/...ts.FromFieldWithAttributes#TestNs.TestVM.ObservableAsPropertyFromObservable.g.verified.cs b/...ts.FromFieldWithAttributes#TestNs.TestVM.ObservableAsPropertyFromObservable.g.verified.cs
@@ -11,7 +11,7 @@ namespace TestNs
     public partial class TestVM
     {
         /// <inheritdoc cref="InternalStateProperty"/>
-        private string _internalStateProperty;
+        private string _internalStateProperty = string.Empty;
 
         /// <inheritdoc cref="_internalStatePropertyHelper"/>
         private ReactiveUI.ObservableAsPropertyHelper<string>? _internalStatePropertyHelper;

diff --git a/...ipleObservableAsProperties#TestNs.TestVM.ObservableAsPropertyFromObservable.g.verified.cs b/...ipleObservableAsProperties#TestNs.TestVM.ObservableAsPropertyFromObservable.g.verified.cs
@@ -20,7 +20,7 @@ public partial class TestVM
         [global::System.Diagnostics.CodeAnalysis.ExcludeFromCodeCoverage]
         public bool HasItemsProperty { get => _hasItemsProperty = _hasItemsPropertyHelper?.Value ?? _hasItemsProperty; }
 /// <inheritdoc cref="DisplayNameProperty"/>
-        private string _displayNameProperty;
+        private string _displayNameProperty = string.Empty;
 
         /// <inheritdoc cref="_displayNamePropertyHelper"/>
         private ReactiveUI.ObservableAsPropertyHelper<string>? _displayNamePropertyHelper;

diff --git a/src/ReactiveUI.SourceGenerator.Tests/UnitTests/OAPFromObservableGeneratorTests.cs b/src/ReactiveUI.SourceGenerator.Tests/UnitTests/OAPFromObservableGeneratorTests.cs
@@ -314,4 +314,79 @@ public TestVM()
         // Act: Initialize the helper and run the generator. Assert: Verify the generated code.
         return TestHelper.TestPass(sourceCode);
     }
+
+    /// <summary>Tests that an empty string initial value initialises the generated backing field.</summary>
+    /// <returns>
+    /// A task to monitor the async.
+    /// </returns>
+    [Test]
+    public Task FromPartialPropertyWithEmptyStringInitialValue()
+    {
+        // Arrange: Setup the source code that matches the generator input expectations.
+        const string sourceCode = """
+                using System;
+                using ReactiveUI;
+                using ReactiveUI.SourceGenerators;
+                using System.Reactive.Linq;
+                using System.Reactive.Subjects;
+
+                namespace TestNs;
+
+                public partial class TestVM : ReactiveObject
+                {
+                    private readonly Subject<string> _testSubject = new();
+
+                    public TestVM()
+                    {
+                        _pLCActiveHelper = _testSubject.ToProperty(this, nameof(PLCActive));
+                    }
+
+                    [ObservableAsProperty(InitialValue = "")]
+                    public partial string PLCActive { get; }
+                }
+            """;
+
+        // Act: Initialize the helper and run the generator. Assert: Verify the generated code.
+        return TestHelper.TestPass(sourceCode);
+    }
+
+    /// <summary>Tests that a non nullable string without an initial value defaults to an empty string.</summary>
+    /// <returns>
+    /// A task to monitor the async.
+    /// </returns>
+    [Test]
+    public Task FromPartialPropertyWithoutInitialValue()
+    {
+        // Arrange: Setup the source code that matches the generator input expectations.
+        const string sourceCode = """
+                using System;
+                using ReactiveUI;
+                using ReactiveUI.SourceGenerators;
+                using System.Reactive.Linq;
+                using System.Reactive.Subjects;
+
+                namespace TestNs;
+
+                public partial class TestVM : ReactiveObject
+                {
+                    private readonly Subject<string> _testSubject = new();
+                    private readonly Subject<string?> _testNullableSubject = new();
+
+                    public TestVM()
+                    {
+                        _pLCActiveHelper = _testSubject.ToProperty(this, nameof(PLCActive));
+                        _pLCNameHelper = _testNullableSubject.ToProperty(this, nameof(PLCName));
+                    }
+
+                    [ObservableAsProperty]
+                    public partial string PLCActive { get; }
+
+                    [ObservableAsProperty]
+                    public partial string? PLCName { get; }
+                }
+            """;
+
+        // Act: Initialize the helper and run the generator. Assert: Verify the generated code.
+        return TestHelper.TestPass(sourceCode);
+    }
 }
diff --git a/src/ReactiveUI.SourceGenerators.Execute/TestViewModel.Initialization.cs b/src/ReactiveUI.SourceGenerators.Execute/TestViewModel.Initialization.cs
@@ -44,6 +44,11 @@ private ObservableAsPropertyHelper<int> CreateObservableAsPropertyFromPropertyHe
     private ObservableAsPropertyHelper<string?> CreatePlcActiveHelper() =>
         this.WhenAnyValue(x => x.PartialRequiredPropertyTest).ToProperty(this, nameof(PLCActive));
 
+    /// <summary>Creates the helper that projects the PLC status message.</summary>
+    /// <returns>The initialized observable property helper.</returns>
+    private ObservableAsPropertyHelper<string> CreatePlcStatusHelper() =>
+        this.WhenAnyValue(x => x.PLCActive).Select(static x => x ?? string.Empty).ToProperty(this, nameof(PLCStatus));
+
     /// <summary>Creates the helper that projects the PLC port.</summary>
     /// <returns>The initialized observable property helper.</returns>
     private ObservableAsPropertyHelper<int> CreatePlcPortHelper() =>

diff --git a/src/ReactiveUI.SourceGenerators.Execute/TestViewModel.cs b/src/ReactiveUI.SourceGenerators.Execute/TestViewModel.cs
@@ -150,6 +150,7 @@ public TestViewModel()
         _observableAsPropertyTest3PropertyHelper = CreateObservableAsPropertyTest3PropertyHelper();
         _observableAsPropertyFromPropertyHelper = CreateObservableAsPropertyFromPropertyHelper();
         _pLCActiveHelper = CreatePlcActiveHelper();
+        _pLCStatusHelper = CreatePlcStatusHelper();
         _pLCPortHelper = CreatePlcPortHelper();
         _instanceOfPLCHelper = CreatePlcInstanceHelper();
         _referenceTypeObservableProperty = default!;
@@ -254,6 +255,12 @@ public TestViewModel()
     [ObservableAsProperty(InitialValue = "Not Connected")]
     public partial string? PLCActive { get; }
 
+    /// <summary>
+    /// Gets the current PLC status message, initialized with an empty string.
+    /// </summary>
+    [ObservableAsProperty(InitialValue = "")]
+    public partial string PLCStatus { get; }
+
     /// <summary>
     /// Gets the TCP port number used to communicate with the PLC.
     /// </summary>

diff --git a/...tors.Roslyn/ObservableAsProperty/ObservableAsPropertyGenerator{FromObservable}.Execute.cs b/...tors.Roslyn/ObservableAsProperty/ObservableAsPropertyGenerator{FromObservable}.Execute.cs
@@ -401,16 +401,27 @@ private static string GetPropertySyntax(ObservableMethodInfo propertyInfo, strin
     /// <returns>The field initializer suffix.</returns>
     private static string GetInitialValueSyntax(string propertyType, string? initialValue)
     {
-        if (initialValue is null || string.IsNullOrWhiteSpace(initialValue))
+        var isNullableStringProperty = propertyType is "string?"
+            || propertyType.EndsWith("##string?", System.StringComparison.Ordinal);
+        var isStringProperty = isNullableStringProperty
+            || propertyType is "string"
+            || propertyType.EndsWith("##string", System.StringComparison.Ordinal);
+
+        if (isStringProperty)
         {
-            return ";";
+            // A non nullable string field is initialised to an empty string when no initial value is
+            // supplied, so the generated field never holds null. Empty and whitespace values are valid
+            // string literals and are emitted as written.
+            if (initialValue is not null)
+            {
+                return $" = {Microsoft.CodeAnalysis.CSharp.SymbolDisplay.FormatLiteral(initialValue, quote: true)};";
+            }
+
+            return isNullableStringProperty ? ";" : " = string.Empty;";
         }
 
-        var isStringProperty = propertyType is "string" or "string?"
-            || propertyType.EndsWith("##string", System.StringComparison.Ordinal)
-            || propertyType.EndsWith("##string?", System.StringComparison.Ordinal);
-        return isStringProperty
-            ? $" = {Microsoft.CodeAnalysis.CSharp.SymbolDisplay.FormatLiteral(initialValue, quote: true)};"
+        return string.IsNullOrWhiteSpace(initialValue)
+            ? ";"
             : $" = {initialValue};";
     }