Added overload of GetValueOrDefault that takes no param.

To match `Nullable<T>`.

src/Avalonia.Base/Data/BindingValue.cs

@@ -185,6 +185,12 @@ namespace Avalonia.Data
             return new BindingValue<T>(type | BindingValueType.HasValue, value, Error);
         }
 
+        /// <summary>
+        /// Gets the value of the binding value if present, otherwise the default value.
+        /// </summary>
+        /// <returns>The value.</returns>
+        public T GetValueOrDefault() => HasValue ? _value : default;
+
         /// <summary>
         /// Gets the value of the binding value if present, otherwise a default value.
         /// </summary>
@@ -192,6 +198,20 @@ namespace Avalonia.Data
         /// <returns>The value.</returns>
         public T GetValueOrDefault(T defaultValue = default) => HasValue ? _value : defaultValue;
 
+        /// <summary>
+        /// Gets the value if present, otherwise the default value.
+        /// </summary>
+        /// <returns>
+        /// The value if present and of the correct type, `default(TResult)` if the value is
+        /// not present or of an incorrect type.
+        /// </returns>
+        public TResult GetValueOrDefault<TResult>()
+        {
+            return HasValue ?
+                _value is TResult result ? result : default
+                : default;
+        }
+
         /// <summary>
         /// Gets the value of the binding value if present, otherwise a default value.
         /// </summary>

src/Avalonia.Base/Data/Optional.cs

@@ -65,6 +65,12 @@ namespace Avalonia.Data
         /// <inheritdoc/>
         public override string ToString() => HasValue ? _value?.ToString() ?? "(null)" : "(empty)";
 
+        /// <summary>
+        /// Gets the value if present, otherwise the default value.
+        /// </summary>
+        /// <returns>The value.</returns>
+        public T GetValueOrDefault() => HasValue ? _value : default;
+
         /// <summary>
         /// Gets the value if present, otherwise a default value.
         /// </summary>
@@ -72,6 +78,20 @@ namespace Avalonia.Data
         /// <returns>The value.</returns>
         public T GetValueOrDefault(T defaultValue = default) => HasValue ? _value : defaultValue;
 
+        /// <summary>
+        /// Gets the value if present, otherwise the default value.
+        /// </summary>
+        /// <returns>
+        /// The value if present and of the correct type, `default(TResult)` if the value is
+        /// not present or of an incorrect type.
+        /// </returns>
+        public TResult GetValueOrDefault<TResult>()
+        {
+            return HasValue ?
+                _value is TResult result ? result : default
+                : default;
+        }
+
         /// <summary>
         /// Gets the value if present, otherwise a default value.
         /// </summary>