Fix: Make Bind() ViewModel parameters nullable.

src/ReactiveUI.Tests/Platforms/winforms/API/ApiApprovalTests.Winforms.net472.approved.txt

@@ -10,7 +10,7 @@ namespace ReactiveUI.Winforms
     public class ContentControlBindingHook : ReactiveUI.IPropertyBindingHook
     {
         public ContentControlBindingHook() { }
-        public bool ExecuteHook(object source, object target, System.Func<ReactiveUI.IObservedChange<, >[]> getCurrentViewModelProperties, System.Func<ReactiveUI.IObservedChange<, >[]> getCurrentViewProperties, ReactiveUI.BindingDirection direction) { }
+        public bool ExecuteHook(object? source, object target, System.Func<ReactiveUI.IObservedChange<, >[]> getCurrentViewModelProperties, System.Func<ReactiveUI.IObservedChange<, >[]> getCurrentViewProperties, ReactiveUI.BindingDirection direction) { }
     }
     public class CreatesWinformsCommandBinding : ReactiveUI.ICreatesCommandBinding
     {

src/ReactiveUI.Tests/Platforms/winforms/API/ApiApprovalTests.Winforms.netcoreapp3.1.approved.txt

@@ -10,7 +10,7 @@ namespace ReactiveUI.Winforms
     public class ContentControlBindingHook : ReactiveUI.IPropertyBindingHook
     {
         public ContentControlBindingHook() { }
-        public bool ExecuteHook(object source, object target, System.Func<ReactiveUI.IObservedChange<, >[]> getCurrentViewModelProperties, System.Func<ReactiveUI.IObservedChange<, >[]> getCurrentViewProperties, ReactiveUI.BindingDirection direction) { }
+        public bool ExecuteHook(object? source, object target, System.Func<ReactiveUI.IObservedChange<, >[]> getCurrentViewModelProperties, System.Func<ReactiveUI.IObservedChange<, >[]> getCurrentViewProperties, ReactiveUI.BindingDirection direction) { }
     }
     public class CreatesWinformsCommandBinding : ReactiveUI.ICreatesCommandBinding
     {

src/ReactiveUI.Winforms/ContentControlBindingHook.cs

@@ -17,7 +17,7 @@ namespace ReactiveUI.Winforms
     public class ContentControlBindingHook : IPropertyBindingHook
     {
         /// <inheritdoc/>
-        public bool ExecuteHook(object source, object target, Func<IObservedChange<object, object>[]> getCurrentViewModelProperties, Func<IObservedChange<object, object>[]> getCurrentViewProperties, BindingDirection direction)
+        public bool ExecuteHook(object? source, object target, Func<IObservedChange<object, object>[]> getCurrentViewModelProperties, Func<IObservedChange<object, object>[]> getCurrentViewProperties, BindingDirection direction)
         {
             if (getCurrentViewProperties == null)
             {

src/ReactiveUI/Bindings/Property/IPropertyBinderImplementation.cs

@@ -65,7 +65,7 @@ public interface IPropertyBinderImplementation : IEnableLogger
         /// disconnects the binding.
         /// </returns>
         IReactiveBinding<TView, TViewModel, (object? view, bool isViewModel)>? Bind<TViewModel, TView, TVMProp, TVProp, TDontCare>(
-                TViewModel viewModel,
+                TViewModel? viewModel,
                 TView view,
                 Expression<Func<TViewModel, TVMProp>> vmProperty,
                 Expression<Func<TView, TVProp>> viewProperty,
@@ -122,7 +122,7 @@ public interface IPropertyBinderImplementation : IEnableLogger
         /// disconnects the binding.
         /// </returns>
         IReactiveBinding<TView, TViewModel, (object? view, bool isViewModel)>? Bind<TViewModel, TView, TVMProp, TVProp, TDontCare>(
-                TViewModel viewModel,
+                TViewModel? viewModel,
                 TView view,
                 Expression<Func<TViewModel, TVMProp>> vmProperty,
                 Expression<Func<TView, TVProp>> viewProperty,
@@ -172,7 +172,7 @@ public interface IPropertyBinderImplementation : IEnableLogger
         /// There is no registered converter from <typeparamref name="TVMProp"/> to <typeparamref name="TVProp"/>.
         /// </exception>
         IReactiveBinding<TView, TViewModel, TVProp>? OneWayBind<TViewModel, TView, TVMProp, TVProp>(
-                TViewModel viewModel,
+                TViewModel? viewModel,
                 TView view,
                 Expression<Func<TViewModel, TVMProp>> vmProperty,
                 Expression<Func<TView, TVProp>> viewProperty,
@@ -213,7 +213,7 @@ public interface IPropertyBinderImplementation : IEnableLogger
         /// disconnects the binding.
         /// </returns>
         IReactiveBinding<TView, TViewModel, TOut>? OneWayBind<TViewModel, TView, TProp, TOut>(
-            TViewModel viewModel,
+            TViewModel? viewModel,
             TView view,
             Expression<Func<TViewModel, TProp>> vmProperty,
             Expression<Func<TView, TOut>> viewProperty,

src/ReactiveUI/Bindings/Property/PropertyBinderImplementation.cs

@@ -1,4 +1,4 @@
-// Copyright (c) 2019 .NET Foundation and Contributors. All rights reserved.
+// Copyright (c) 2019 .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.
@@ -48,7 +48,7 @@ public class PropertyBinderImplementation : IPropertyBinderImplementation
 
         /// <inheritdoc />
         public IReactiveBinding<TView, TViewModel, (object? view, bool isViewModel)> Bind<TViewModel, TView, TVMProp, TVProp, TDontCare>(
-                TViewModel viewModel,
+                TViewModel? viewModel,
                 TView view,
                 Expression<Func<TViewModel, TVMProp>> vmProperty,
                 Expression<Func<TView, TVProp>> viewProperty,
@@ -91,7 +91,7 @@ bool ViewToVmFunc(TVProp vValue, out TVMProp vmValue)
 
         /// <inheritdoc />
         public IReactiveBinding<TView, TViewModel, (object? view, bool isViewModel)>? Bind<TViewModel, TView, TVMProp, TVProp, TDontCare>(
-                TViewModel viewModel,
+                TViewModel? viewModel,
                 TView view,
                 Expression<Func<TViewModel, TVMProp>> vmProperty,
                 Expression<Func<TView, TVProp>> viewProperty,
@@ -138,7 +138,7 @@ bool ViewToVmFunc(TVProp vValue, out TVMProp vmValue)
 
         /// <inheritdoc />
         public IReactiveBinding<TView, TViewModel, TVProp>? OneWayBind<TViewModel, TView, TVMProp, TVProp>(
-                TViewModel viewModel,
+                TViewModel? viewModel,
                 TView view,
                 Expression<Func<TViewModel, TVMProp>> vmProperty,
                 Expression<Func<TView, TVProp>> viewProperty,
@@ -191,7 +191,7 @@ bool ViewToVmFunc(TVProp vValue, out TVMProp vmValue)
 
         /// <inheritdoc />
         public IReactiveBinding<TView, TViewModel, TOut>? OneWayBind<TViewModel, TView, TProp, TOut>(
-            TViewModel viewModel,
+            TViewModel? viewModel,
             TView view,
             Expression<Func<TViewModel, TProp>> vmProperty,
             Expression<Func<TView, TOut>> viewProperty,
@@ -353,7 +353,7 @@ public IDisposable BindTo<TValue, TTarget, TTValue>(
             return (setObservable.Subscribe(_ => { }, ex => this.Log().Error(ex, $"{viewExpression} Binding received an Exception!")), setObservable);
         }
 
-        private bool EvalBindingHooks<TViewModel, TView>(TViewModel viewModel, TView view, Expression vmExpression, Expression viewExpression, BindingDirection direction)
+        private bool EvalBindingHooks<TViewModel, TView>(TViewModel? viewModel, TView view, Expression vmExpression, Expression viewExpression, BindingDirection direction)
             where TViewModel : class
         {
             var hooks = Locator.Current.GetServices<IPropertyBindingHook>();
@@ -376,7 +376,7 @@ private bool EvalBindingHooks<TViewModel, TView>(TViewModel viewModel, TView vie
             {
                 vmFetcher = () => new IObservedChange<object, object?>[]
                 {
-                    new ObservedChange<object, object>(null!, null!, viewModel)
+                    new ObservedChange<object, object?>(null!, null!, viewModel)
                 };
             }
 
@@ -400,7 +400,7 @@ private bool EvalBindingHooks<TViewModel, TView>(TViewModel viewModel, TView vie
         }
 
         private IReactiveBinding<TView, TViewModel, (object? view, bool isViewModel)> BindImpl<TViewModel, TView, TVMProp, TVProp, TDontCare>(
-            TViewModel viewModel,
+            TViewModel? viewModel,
             TView view,
             Expression<Func<TViewModel, TVMProp>> vmProperty,
             Expression<Func<TView, TVProp>> viewProperty,

src/ReactiveUI/Bindings/Property/PropertyBindingMixins.cs

@@ -59,7 +59,7 @@ static PropertyBindingMixins()
         /// </returns>
         public static IReactiveBinding<TView, TViewModel, (object? view, bool isViewModel)>? Bind<TViewModel, TView, TVMProp, TVProp>(
                 this TView view,
-                TViewModel viewModel,
+                TViewModel? viewModel,
                 Expression<Func<TViewModel, TVMProp>> vmProperty,
                 Expression<Func<TView, TVProp>> viewProperty,
                 object? conversionHint = null,
@@ -126,7 +126,7 @@ static PropertyBindingMixins()
         /// </returns>
         public static IReactiveBinding<TView, TViewModel, (object? view, bool isViewModel)>? Bind<TViewModel, TView, TVMProp, TVProp, TDontCare>(
                 this TView view,
-                TViewModel viewModel,
+                TViewModel? viewModel,
                 Expression<Func<TViewModel, TVMProp>> vmProperty,
                 Expression<Func<TView, TVProp>> viewProperty,
                 IObservable<TDontCare>? signalViewUpdate,
@@ -170,7 +170,7 @@ static PropertyBindingMixins()
         /// </returns>
         public static IReactiveBinding<TView, TViewModel, (object? view, bool isViewModel)>? Bind<TViewModel, TView, TVMProp, TVProp>(
             this TView view,
-            TViewModel viewModel,
+            TViewModel? viewModel,
             Expression<Func<TViewModel, TVMProp>> vmProperty,
             Expression<Func<TView, TVProp>> viewProperty,
             Func<TVMProp, TVProp> vmToViewConverter,
@@ -220,7 +220,7 @@ static PropertyBindingMixins()
         /// </returns>
         public static IReactiveBinding<TView, TViewModel, (object? view, bool isViewModel)>? Bind<TViewModel, TView, TVMProp, TVProp, TDontCare>(
             this TView view,
-            TViewModel viewModel,
+            TViewModel? viewModel,
             Expression<Func<TViewModel, TVMProp>> vmProperty,
             Expression<Func<TView, TVProp>> viewProperty,
             IObservable<TDontCare>? signalViewUpdate,
@@ -270,7 +270,7 @@ static PropertyBindingMixins()
         /// </returns>
         public static IReactiveBinding<TView, TViewModel, TVProp>? OneWayBind<TViewModel, TView, TVMProp, TVProp>(
                 this TView view,
-                TViewModel viewModel,
+                TViewModel? viewModel,
                 Expression<Func<TViewModel, TVMProp>> vmProperty,
                 Expression<Func<TView, TVProp>> viewProperty,
                 object? conversionHint = null,
@@ -319,7 +319,7 @@ static PropertyBindingMixins()
         /// </returns>
         public static IReactiveBinding<TView, TViewModel, TOut>? OneWayBind<TViewModel, TView, TProp, TOut>(
                 this TView view,
-                TViewModel viewModel,
+                TViewModel? viewModel,
                 Expression<Func<TViewModel, TProp>> vmProperty,
                 Expression<Func<TView, TOut>> viewProperty,
                 Func<TProp, TOut> selector)

src/ReactiveUI/Bindings/Reactive/IReactiveBinding.cs

@@ -24,7 +24,7 @@ public interface IReactiveBinding<out TView, out TViewModel, out TValue> : IDisp
         /// <summary>
         /// Gets the instance of the view model this binding is applied to.
         /// </summary>
-        TViewModel ViewModel { get; }
+        TViewModel? ViewModel { get; }
 
         /// <summary>
         /// Gets an expression representing the propertyon the viewmodel bound to the view.

src/ReactiveUI/Bindings/Reactive/ReactiveBinding.cs

@@ -16,7 +16,7 @@ internal class ReactiveBinding<TView, TViewModel, TValue> : IReactiveBinding<TVi
 
         public ReactiveBinding(
             TView view,
-            TViewModel viewModel,
+            TViewModel? viewModel,
             Expression viewExpression,
             Expression viewModelExpression,
             IObservable<TValue> changed,
@@ -34,7 +34,7 @@ public ReactiveBinding(
         }
 
         /// <inheritdoc />
-        public TViewModel ViewModel { get; }
+        public TViewModel? ViewModel { get; }
 
         /// <inheritdoc />
         public Expression ViewModelExpression { get; }

src/ReactiveUI/Expression/Reflection.cs

@@ -433,7 +433,7 @@ public static bool IsStatic(this PropertyInfo item)
         }
 
         [SuppressMessage("Microsoft.Performance", "CA1801", Justification = "TViewModel used to help generic calling.")]
-        internal static IObservable<object> ViewModelWhenAnyValue<TView, TViewModel>(TViewModel viewModel, TView view, Expression expression)
+        internal static IObservable<object> ViewModelWhenAnyValue<TView, TViewModel>(TViewModel? viewModel, TView view, Expression expression)
             where TView : class, IViewFor
             where TViewModel : class
         {

src/ReactiveUI/Interfaces/IPropertyBindingHook.cs

@@ -23,7 +23,7 @@ public interface IPropertyBindingHook
         /// <param name="getCurrentViewProperties">Get current view properties.</param>
         /// <param name="direction">The Binding direction.</param>
         bool ExecuteHook(
-            object source,
+            object? source,
             object target,
             Func<IObservedChange<object, object>[]> getCurrentViewModelProperties,
             Func<IObservedChange<object, object>[]> getCurrentViewProperties,

src/ReactiveUI/Platforms/windows-common/AutoDataTemplateBindingHook.cs

@@ -56,7 +56,7 @@ public class AutoDataTemplateBindingHook : IPropertyBindingHook
         });
 
         /// <inheritdoc/>
-        public bool ExecuteHook(object source, object target, Func<IObservedChange<object, object>[]> getCurrentViewModelProperties, Func<IObservedChange<object, object>[]> getCurrentViewProperties, BindingDirection direction)
+        public bool ExecuteHook(object? source, object target, Func<IObservedChange<object, object>[]> getCurrentViewModelProperties, Func<IObservedChange<object, object>[]> getCurrentViewProperties, BindingDirection direction)
         {
             if (getCurrentViewProperties == null)
             {