Why does MSBuild fails to find my constant in Directory.Build.props?

Question

I am trying to use a Directory.Build.props file to define a simple preprocessor constant for to my solution.

For testing purposes I created a console application (C#) and added a Directoy.Build.props file right next to the generated .csproj file:

<Project>
   <PropertyGroup>
       <DefineConstants>TEST1</DefineConstants>
   </PropertyGroup>
</Project>

I also manually edited the .csproj itself to test the <DefineConstants> actually works:

<DefineConstants>DEBUG;TRACE;TEST2;</DefineConstants>

Now the actual code:

static void Main(string[] args)
{
#if TEST1
    // NOT WORKING!
    Console.WriteLine("<DefineConstants>TEST1</DefineConstants> in Directory.Build.props"); // NOT WORKING!
#else
    Console.WriteLine("failed TEST1 in Directory.Build.props");
#endif

#if TEST2
     // WORKS!
     Console.WriteLine("<DefineConstants>TEST2</DefineConstants> in csproj"); // WORKS!
#else
     Console.WriteLine("failed TEST2 in csproj");
#endif

    }

So why does MSBuild fails to find my constant in Directory.Build.props?

Have a look at https://stackoverflow.com/questions/36793499/defineconstants-in-addition-to-existing-ones-in-csproj, I'm guessing that the same thing applies with Directory.build.props. — Karl-Johan Sjögren, Jan 28 '21 at 12:13
@Karl-JohanSjögren OH wow! that did it! but is there a way it will happen automatically without manually editing the csproj? — Elad Maimoni, Jan 28 '21 at 12:16

score 3 · Accepted Answer · answered Jan 29 '21 at 06:07

You should note that Directory.Build.props file is imported at the begin of the csproj file. In your side, because the imported property is too early to be overridden, it is overridden by defineConstants in CSProj.

So Directory.Build.props is usually used to define global properties while Directory.Build.targets is to overwrite the properties.

Check this link.

So you should use to overwrite the property.

Solutions

I have two solutions for you:

1) rename your Directory.Build.props to Directory.Build.targets and ensure these on the file:

<Project>
   <PropertyGroup>
       <DefineConstants>$(DefineConstants)TEST1</DefineConstants>
   </PropertyGroup>
</Project>

Make sure that csproj file like this:

<PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">
    <PlatformTarget>AnyCPU</PlatformTarget>
    <DebugSymbols>true</DebugSymbols>
    <DebugType>full</DebugType>
    <Optimize>false</Optimize>
    <OutputPath>bin\Debug\</OutputPath>
    <DefineConstants>DEBUG;TRACE;TEST2;</DefineConstants>
    <ErrorReport>prompt</ErrorReport>
    <WarningLevel>4</WarningLevel>
  </PropertyGroup>

2) remain the file as Directory.Build.props and keep its content as:

<Project>
   <PropertyGroup>
       <DefineConstants>TEST1</DefineConstants>
   </PropertyGroup>
</Project>

change your csproj file to:

<PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">
    <PlatformTarget>AnyCPU</PlatformTarget>
    <DebugSymbols>true</DebugSymbols>
    <DebugType>full</DebugType>
    <Optimize>false</Optimize>
    <OutputPath>bin\Debug\</OutputPath>
    <DefineConstants>DEBUG;TRACE;TEST2;$(DefineConstants)</DefineConstants>
    <ErrorReport>prompt</ErrorReport>
    <WarningLevel>4</WarningLevel>
  </PropertyGroup>

thanks! I might end up using the first option you suggested. — Elad Maimoni, Jan 29 '21 at 09:08

Why does MSBuild fails to find my constant in Directory.Build.props?

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