diff --git a/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/Marshalling/AnsiStringMarshaller.cs b/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/Marshalling/AnsiStringMarshaller.cs
index d8e0576fc1bf2d..5522fa8bb55edb 100644
--- a/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/Marshalling/AnsiStringMarshaller.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/Marshalling/AnsiStringMarshaller.cs
@@ -13,8 +13,8 @@ namespace System.Runtime.InteropServices.Marshalling
         Features = CustomTypeMarshallerFeatures.UnmanagedResources | CustomTypeMarshallerFeatures.TwoStageMarshalling | CustomTypeMarshallerFeatures.CallerAllocatedBuffer)]
     public unsafe ref struct AnsiStringMarshaller
     {
-        private byte* _allocated;
-        private readonly Span<byte> _span;
+        private byte* _nativeValue;
+        private bool _allocated;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="AnsiStringMarshaller"/>.
@@ -36,25 +36,29 @@ public AnsiStringMarshaller(string? str)
         /// </remarks>
         public AnsiStringMarshaller(string? str, Span<byte> buffer)
         {
-            _allocated = null;
+            _allocated = false;
+
             if (str is null)
             {
-                _span = default;
+                _nativeValue = null;
                 return;
             }
 
-            // + 1 for null terminator
-            int maxByteCount = (str.Length + 1) * Marshal.SystemMaxDBCSCharSize + 1;
-            if (buffer.Length >= maxByteCount)
-            {
-                Marshal.StringToAnsiString(str, (byte*)Unsafe.AsPointer(ref MemoryMarshal.GetReference(buffer)), buffer.Length);
-                _span = buffer;
-            }
-            else
+            // >= for null terminator
+            if ((long)Marshal.SystemMaxDBCSCharSize * str.Length >= buffer.Length)
             {
-                _allocated = (byte*)Marshal.StringToCoTaskMemAnsi(str);
-                _span = default;
+                // Calculate accurate byte count when the provided stack-allocated buffer is not sufficient
+                int exactByteCount = Marshal.GetAnsiStringByteCount(str); // Includes null terminator
+                if (exactByteCount > buffer.Length)
+                {
+                    buffer = new Span<byte>((byte*)Marshal.AllocCoTaskMem(exactByteCount), exactByteCount);
+                    _allocated = true;
+                }
             }
+
+            _nativeValue = (byte*)Unsafe.AsPointer(ref MemoryMarshal.GetReference(buffer));
+
+            Marshal.GetAnsiStringBytes(str, buffer); // Includes null terminator
         }
 
         /// <summary>
@@ -63,7 +67,7 @@ public AnsiStringMarshaller(string? str, Span<byte> buffer)
         /// <remarks>
         /// <seealso cref="CustomTypeMarshallerFeatures.TwoStageMarshalling"/>
         /// </remarks>
-        public byte* ToNativeValue() => _allocated != null ? _allocated : (byte*)Unsafe.AsPointer(ref MemoryMarshal.GetReference(_span));
+        public byte* ToNativeValue() => _nativeValue;
 
         /// <summary>
         /// Sets the native value representing the string.
@@ -72,7 +76,11 @@ public AnsiStringMarshaller(string? str, Span<byte> buffer)
         /// <remarks>
         /// <seealso cref="CustomTypeMarshallerFeatures.TwoStageMarshalling"/>
         /// </remarks>
-        public void FromNativeValue(byte* value) => _allocated = value;
+        public void FromNativeValue(byte* value)
+        {
+            _nativeValue = value;
+            _allocated = true;
+        }
 
         /// <summary>
         /// Returns the managed string.
@@ -80,7 +88,7 @@ public AnsiStringMarshaller(string? str, Span<byte> buffer)
         /// <remarks>
         /// <seealso cref="CustomTypeMarshallerDirection.Out"/>
         /// </remarks>
-        public string? ToManaged() => _allocated == null ? null : new string((sbyte*)_allocated);
+        public string? ToManaged() => Marshal.PtrToStringAnsi((IntPtr)_nativeValue);
 
         /// <summary>
         /// Frees native resources.
@@ -90,8 +98,8 @@ public AnsiStringMarshaller(string? str, Span<byte> buffer)
         /// </remarks>
         public void FreeNative()
         {
-            if (_allocated != null)
-                Marshal.FreeCoTaskMem((IntPtr)_allocated);
+            if (_allocated)
+                Marshal.FreeCoTaskMem((IntPtr)_nativeValue);
         }
     }
 }
diff --git a/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/Marshalling/Utf16StringMarshaller.cs b/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/Marshalling/Utf16StringMarshaller.cs
index e5220a1571f7c1..7d77bedc19ff51 100644
--- a/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/Marshalling/Utf16StringMarshaller.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/Marshalling/Utf16StringMarshaller.cs
@@ -9,64 +9,33 @@ namespace System.Runtime.InteropServices.Marshalling
     /// Marshaller for UTF-16 strings
     /// </summary>
     [CLSCompliant(false)]
-    [CustomTypeMarshaller(typeof(string), BufferSize = 0x100,
-        Features = CustomTypeMarshallerFeatures.UnmanagedResources | CustomTypeMarshallerFeatures.TwoStageMarshalling | CustomTypeMarshallerFeatures.CallerAllocatedBuffer)]
+    [CustomTypeMarshaller(typeof(string),
+        Features = CustomTypeMarshallerFeatures.UnmanagedResources | CustomTypeMarshallerFeatures.TwoStageMarshalling)]
     public unsafe ref struct Utf16StringMarshaller
     {
-        private ushort* _allocated;
-        private readonly Span<ushort> _span;
+        private ushort* _nativeValue;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="Utf16StringMarshaller"/>.
         /// </summary>
-        /// <param name="str">The string to marshal.</param>
-        public Utf16StringMarshaller(string? str)
-            : this(str, default)
-        {
-        }
-
-        /// <summary>
-        /// Initializes a new instance of the <see cref="Utf16StringMarshaller"/>.
-        /// </summary>
-        /// <param name="str">The string to marshal.</param>
-        /// <param name="buffer">Buffer that may be used for marshalling.</param>
         /// <remarks>
-        /// The <paramref name="buffer"/> must not be movable - that is, it should not be
-        /// on the managed heap or it should be pinned.
-        /// <seealso cref="CustomTypeMarshallerFeatures.CallerAllocatedBuffer"/>
+        /// The caller allocated constructor option is not provided because
+        /// pinning should be preferred for UTF-16 scenarios.
         /// </remarks>
-        public Utf16StringMarshaller(string? str, Span<ushort> buffer)
+        /// <param name="str">The string to marshal.</param>
+        public Utf16StringMarshaller(string? str)
         {
-            _allocated = null;
             if (str is null)
             {
-                _span = default;
+                _nativeValue = null;
                 return;
             }
 
             // + 1 for null terminator
-            if (buffer.Length >= str.Length + 1)
-            {
-                _span = buffer;
-                str.CopyTo(MemoryMarshal.Cast<ushort, char>(buffer));
-                _span[str.Length] = '\0'; // null-terminate
-            }
-            else
-            {
-                _allocated = (ushort*)Marshal.StringToCoTaskMemUni(str);
-                _span = default;
-            }
-        }
-
-        /// <summary>
-        /// Returns a reference to the marshalled string.
-        /// </summary>
-        public ref ushort GetPinnableReference()
-        {
-            if (_allocated != null)
-                return ref Unsafe.AsRef<ushort>(_allocated);
+            _nativeValue =  (ushort*)Marshal.AllocCoTaskMem((str.Length + 1) * sizeof(ushort));
 
-            return ref _span.GetPinnableReference();
+            str.CopyTo(new Span<char>(_nativeValue, str.Length));
+            _nativeValue[str.Length] = '\0'; // null-terminate
         }
 
         /// <summary>
@@ -75,7 +44,7 @@ public ref ushort GetPinnableReference()
         /// <remarks>
         /// <seealso cref="CustomTypeMarshallerFeatures.TwoStageMarshalling"/>
         /// </remarks>
-        public ushort* ToNativeValue() => _allocated != null ? _allocated : (ushort*)Unsafe.AsPointer(ref MemoryMarshal.GetReference(_span));
+        public ushort* ToNativeValue() => _nativeValue;
 
         /// <summary>
         /// Sets the native value representing the string.
@@ -84,7 +53,7 @@ public ref ushort GetPinnableReference()
         /// <remarks>
         /// <seealso cref="CustomTypeMarshallerFeatures.TwoStageMarshalling"/>
         /// </remarks>
-        public void FromNativeValue(ushort* value) => _allocated = value;
+        public void FromNativeValue(ushort* value) => _nativeValue = value;
 
         /// <summary>
         /// Returns the managed string.
@@ -92,7 +61,7 @@ public ref ushort GetPinnableReference()
         /// <remarks>
         /// <seealso cref="CustomTypeMarshallerDirection.Out"/>
         /// </remarks>
-        public string? ToManaged() => _allocated == null ? null : new string((char*)_allocated);
+        public string? ToManaged() => Marshal.PtrToStringUni((IntPtr)_nativeValue);
 
         /// <summary>
         /// Frees native resources.
@@ -102,8 +71,7 @@ public ref ushort GetPinnableReference()
         /// </remarks>
         public void FreeNative()
         {
-            if (_allocated != null)
-                Marshal.FreeCoTaskMem((IntPtr)_allocated);
+            Marshal.FreeCoTaskMem((IntPtr)_nativeValue);
         }
     }
 }
diff --git a/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/Marshalling/Utf8StringMarshaller.cs b/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/Marshalling/Utf8StringMarshaller.cs
index 8737d0de3928be..be37a40586f926 100644
--- a/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/Marshalling/Utf8StringMarshaller.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/Marshalling/Utf8StringMarshaller.cs
@@ -14,8 +14,8 @@ namespace System.Runtime.InteropServices.Marshalling
         Features = CustomTypeMarshallerFeatures.UnmanagedResources | CustomTypeMarshallerFeatures.TwoStageMarshalling | CustomTypeMarshallerFeatures.CallerAllocatedBuffer)]
     public unsafe ref struct Utf8StringMarshaller
     {
-        private byte* _allocated;
-        private readonly Span<byte> _span;
+        private byte* _nativeValue;
+        private bool _allocated;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="Utf8StringMarshaller"/>.
@@ -37,32 +37,32 @@ public Utf8StringMarshaller(string? str)
         /// </remarks>
         public Utf8StringMarshaller(string? str, Span<byte> buffer)
         {
-            _allocated = null;
+            _allocated = false;
+
             if (str is null)
             {
-                _span = default;
+                _nativeValue = null;
                 return;
             }
 
-            // + 1 for null terminator
-            int maxByteCount =  Encoding.UTF8.GetMaxByteCount(str.Length) + 1;
-            if (buffer.Length >= maxByteCount)
-            {
-                int byteCount = Encoding.UTF8.GetBytes(str, buffer);
-                buffer[byteCount] = 0; // null-terminate
-                _span = buffer;
-            }
-            else
+            const int MaxUtf8BytesPerChar = 3;
+
+            // >= for null terminator
+            if ((long)MaxUtf8BytesPerChar * str.Length >= buffer.Length)
             {
-                _allocated = (byte*)Marshal.AllocCoTaskMem(maxByteCount);
-                int byteCount;
-                fixed (char* ptr = str)
+                // Calculate accurate byte count when the provided stack-allocated buffer is not sufficient
+                int exactByteCount = checked(Encoding.UTF8.GetByteCount(str) + 1); // + 1 for null terminator
+                if (exactByteCount > buffer.Length)
                 {
-                    byteCount = Encoding.UTF8.GetBytes(ptr, str.Length, _allocated, maxByteCount);
+                    buffer = new Span<byte>((byte*)Marshal.AllocCoTaskMem(exactByteCount), exactByteCount);
+                    _allocated = true;
                 }
-                _allocated[byteCount] = 0; // null-terminate
-                _span = default;
             }
+
+            _nativeValue = (byte*)Unsafe.AsPointer(ref MemoryMarshal.GetReference(buffer));
+
+            int byteCount = Encoding.UTF8.GetBytes(str, buffer);
+            buffer[byteCount] = 0; // null-terminate
         }
 
         /// <summary>
@@ -71,7 +71,7 @@ public Utf8StringMarshaller(string? str, Span<byte> buffer)
         /// <remarks>
         /// <seealso cref="CustomTypeMarshallerFeatures.TwoStageMarshalling"/>
         /// </remarks>
-        public byte* ToNativeValue() => _allocated != null ? _allocated : (byte*)Unsafe.AsPointer(ref MemoryMarshal.GetReference(_span));
+        public byte* ToNativeValue() => _nativeValue;
 
         /// <summary>
         /// Sets the native value representing the string.
@@ -80,7 +80,11 @@ public Utf8StringMarshaller(string? str, Span<byte> buffer)
         /// <remarks>
         /// <seealso cref="CustomTypeMarshallerFeatures.TwoStageMarshalling"/>
         /// </remarks>
-        public void FromNativeValue(byte* value) => _allocated = value;
+        public void FromNativeValue(byte* value)
+        {
+            _nativeValue = value;
+            _allocated = true;
+        }
 
         /// <summary>
         /// Returns the managed string.
@@ -88,7 +92,7 @@ public Utf8StringMarshaller(string? str, Span<byte> buffer)
         /// <remarks>
         /// <seealso cref="CustomTypeMarshallerDirection.Out"/>
         /// </remarks>
-        public string? ToManaged() => _allocated == null ? null : Marshal.PtrToStringUTF8((IntPtr)_allocated);
+        public string? ToManaged() => Marshal.PtrToStringUTF8((IntPtr)_nativeValue);
 
         /// <summary>
         /// Frees native resources.
@@ -98,8 +102,8 @@ public Utf8StringMarshaller(string? str, Span<byte> buffer)
         /// </remarks>
         public void FreeNative()
         {
-            if (_allocated != null)
-                Marshal.FreeCoTaskMem((IntPtr)_allocated);
+            if (_allocated)
+                Marshal.FreeCoTaskMem((IntPtr)_nativeValue);
         }
     }
 }
diff --git a/src/libraries/System.Runtime.InteropServices/ref/System.Runtime.InteropServices.cs b/src/libraries/System.Runtime.InteropServices/ref/System.Runtime.InteropServices.cs
index 898b96ba95ea81..6c196e7f100621 100644
--- a/src/libraries/System.Runtime.InteropServices/ref/System.Runtime.InteropServices.cs
+++ b/src/libraries/System.Runtime.InteropServices/ref/System.Runtime.InteropServices.cs
@@ -2187,15 +2187,12 @@ public void FromNativeValue(byte* value) { }
         public void FreeNative() { }
     }
     [System.CLSCompliant(false)]
-    [System.Runtime.InteropServices.Marshalling.CustomTypeMarshallerAttribute(typeof(string), BufferSize = 0x100,
+    [System.Runtime.InteropServices.Marshalling.CustomTypeMarshallerAttribute(typeof(string),
         Features = System.Runtime.InteropServices.Marshalling.CustomTypeMarshallerFeatures.UnmanagedResources
-            | System.Runtime.InteropServices.Marshalling.CustomTypeMarshallerFeatures.CallerAllocatedBuffer
             | System.Runtime.InteropServices.Marshalling.CustomTypeMarshallerFeatures.TwoStageMarshalling )]
     public unsafe ref struct Utf16StringMarshaller
     {
         public Utf16StringMarshaller(string? str) { }
-        public Utf16StringMarshaller(string? str, System.Span<ushort> buffer) { }
-        public ref ushort GetPinnableReference() { throw null; }
         public ushort* ToNativeValue() { throw null; }
         public void FromNativeValue(ushort* value) { }
         public string? ToManaged() { throw null; }