Fixed [ReadOnly]Span2D<T>.TryGetSpan when no runtime support

Closes #3947

Author: Sergio0694

Microsoft.Toolkit.HighPerformance/Memory/ReadOnlySpan2D{T}.cs

@@ -906,7 +906,10 @@ public bool TryGetSpan(out ReadOnlySpan<T> span)
                 // Without Span<T> runtime support, we can only get a Span<T> from a T[] instance
                 if (this.instance.GetType() == typeof(T[]))
                 {
-                    span = Unsafe.As<T[]>(this.instance).AsSpan((int)this.offset, (int)Length);
+                    T[] array = Unsafe.As<T[]>(this.instance)!;
+                    int index = array.AsSpan().IndexOf(ref ObjectMarshal.DangerousGetObjectDataReferenceAt<T>(array, this.offset));
+
+                    span = array.AsSpan(index, (int)Length);
 
                     return true;
                 }

Microsoft.Toolkit.HighPerformance/Memory/Span2D{T}.cs

@@ -1062,7 +1062,10 @@ public bool TryGetSpan(out Span<T> span)
                 // Without Span<T> runtime support, we can only get a Span<T> from a T[] instance
                 if (this.Instance.GetType() == typeof(T[]))
                 {
-                    span = Unsafe.As<T[]>(this.Instance).AsSpan((int)this.Offset, (int)Length);
+                    T[] array = Unsafe.As<T[]>(this.Instance)!;
+                    int index = array.AsSpan().IndexOf(ref ObjectMarshal.DangerousGetObjectDataReferenceAt<T>(array, this.Offset));
+
+                    span = array.AsSpan(index, (int)Length);
 
                     return true;
                 }

UnitTests/UnitTests.HighPerformance.Shared/Memory/Test_ReadOnlySpan2D{T}.cs

@@ -605,7 +605,7 @@ public void Test_ReadOnlySpan2DT_TryGetSpan_From1DArray_2()
         {
             int[] array = { 1, 2, 3, 4, 5, 6, 7, 8, 9 };
 
-            ReadOnlySpan2D<int> span2d = new ReadOnlySpan2D<int>(array, 3, 3).Slice(1, 0, 2, 3);
+            ReadOnlySpan2D<int> span2d = new ReadOnlySpan2D<int>(array, 3, 3).Slice(1, 0, 3, 2);
 
             bool success = span2d.TryGetSpan(out ReadOnlySpan<int> span);
 
@@ -620,7 +620,7 @@ public void Test_ReadOnlySpan2DT_TryGetSpan_From1DArray_3()
         {
             int[] array = { 1, 2, 3, 4, 5, 6, 7, 8, 9 };
 
-            ReadOnlySpan2D<int> span2d = new ReadOnlySpan2D<int>(array, 3, 3).Slice(0, 1, 3, 2);
+            ReadOnlySpan2D<int> span2d = new ReadOnlySpan2D<int>(array, 3, 3).Slice(0, 1, 2, 3);
 
             bool success = span2d.TryGetSpan(out ReadOnlySpan<int> span);
 

UnitTests/UnitTests.HighPerformance.Shared/Memory/Test_Span2D{T}.cs

@@ -803,6 +803,21 @@ public void Test_Span2DT_TryGetSpan_From1DArray_3()
             Assert.AreEqual(span.Length, 0);
         }
 
+        // See https://github.com/windows-toolkit/WindowsCommunityToolkit/issues/3947
+        [TestCategory("Span2DT")]
+        [TestMethod]
+        public void Test_Span2DT_TryGetSpan_From1DArray_4()
+        {
+            int[] array = new int[128];
+            Span2D<int> span2d = new Span2D<int>(array, 8, 16);
+
+            bool success = span2d.TryGetSpan(out Span<int> span);
+
+            Assert.IsTrue(success);
+            Assert.AreEqual(span.Length, span2d.Length);
+            Assert.IsTrue(Unsafe.AreSame(ref array[0], ref span[0]));
+        }
+
         [TestCategory("Span2DT")]
         [TestMethod]
         public void Test_Span2DT_TryGetSpan_From2DArray_1()