**Problem:**

* C#/msbuild incremental builds are broken.
* Grpc prebuilt binaries are always copied to output.

**Solution:**
* Change from `Always` to `PreserveNewest`; i.e. only copy if binaries are older.

**Repro:**
* Setup:
    * Create a CSPROJ (e.g. in Visual Studio 2019).
    * Add Grpc NuGet package (at least version 2.29.0).
    * Build CSPROJ.
    * Rebuild CSPROJ.
* Observation:
    * Rebuild results in regeneration CSPROJ output.
* Expected:
    * output should not be rebuilt; i.e. without change to sources.

**Side Notes:**
* Not sure if new updates are being shipped for mono.
* Similar issue exists in the mono build.
* Fixed it anyway.

src/csharp/Grpc.Core/build/MonoAndroid10/Grpc.Core.targets

@@ -3,21 +3,21 @@
 
   <ItemGroup Condition="'$(TargetFrameworkIdentifier)' == 'MonoAndroid'">
     <AndroidNativeLibrary Include="$(MSBuildThisFileDirectory)..\..\native\android\arm64-v8a\libgrpc_csharp_ext.so">
-      <CopyToOutputDirectory>Always</CopyToOutputDirectory>
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
       <Abi>arm64-v8a</Abi>
     </AndroidNativeLibrary>
   </ItemGroup>
 
   <ItemGroup Condition="'$(TargetFrameworkIdentifier)' == 'MonoAndroid'">
     <AndroidNativeLibrary Include="$(MSBuildThisFileDirectory)..\..\native\android\armeabi-v7a\libgrpc_csharp_ext.so">
-      <CopyToOutputDirectory>Always</CopyToOutputDirectory>
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
       <Abi>armeabi-v7a</Abi>
     </AndroidNativeLibrary>
   </ItemGroup>
 
   <ItemGroup Condition="'$(TargetFrameworkIdentifier)' == 'MonoAndroid'">
     <AndroidNativeLibrary Include="$(MSBuildThisFileDirectory)..\..\native\android\x86\libgrpc_csharp_ext.so">
-      <CopyToOutputDirectory>Always</CopyToOutputDirectory>
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
       <Abi>x86</Abi>
     </AndroidNativeLibrary>
   </ItemGroup>

src/csharp/Grpc.Core/build/net45/Grpc.Core.targets

@@ -11,32 +11,32 @@
   -->
   <ItemGroup Condition="'$(Grpc_SkipNativeLibsCopy)' != 'true'">
     <Content Include="$(MSBuildThisFileDirectory)..\..\runtimes\win\native\grpc_csharp_ext.x86.dll">
-      <CopyToOutputDirectory>Always</CopyToOutputDirectory>
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
       <Link>grpc_csharp_ext.x86.dll</Link>
       <Visible>false</Visible>
     </Content>
     <Content Include="$(MSBuildThisFileDirectory)..\..\runtimes\win\native\grpc_csharp_ext.x64.dll">
-      <CopyToOutputDirectory>Always</CopyToOutputDirectory>
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
       <Link>grpc_csharp_ext.x64.dll</Link>
       <Visible>false</Visible>
     </Content>
     <Content Include="$(MSBuildThisFileDirectory)..\..\runtimes\linux\native\libgrpc_csharp_ext.x86.so">
-      <CopyToOutputDirectory>Always</CopyToOutputDirectory>
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
       <Link>libgrpc_csharp_ext.x86.so</Link>
       <Visible>false</Visible>
     </Content>
     <Content Include="$(MSBuildThisFileDirectory)..\..\runtimes\linux\native\libgrpc_csharp_ext.x64.so">
-      <CopyToOutputDirectory>Always</CopyToOutputDirectory>
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
       <Link>libgrpc_csharp_ext.x64.so</Link>
       <Visible>false</Visible>
     </Content>
     <Content Include="$(MSBuildThisFileDirectory)..\..\runtimes\osx\native\libgrpc_csharp_ext.x86.dylib">
-      <CopyToOutputDirectory>Always</CopyToOutputDirectory>
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
       <Link>libgrpc_csharp_ext.x86.dylib</Link>
       <Visible>false</Visible>
     </Content>
     <Content Include="$(MSBuildThisFileDirectory)..\..\runtimes\osx\native\libgrpc_csharp_ext.x64.dylib">
-      <CopyToOutputDirectory>Always</CopyToOutputDirectory>
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
       <Link>libgrpc_csharp_ext.x64.dylib</Link>
       <Visible>false</Visible>
     </Content>