DefineConstants works with Run command, but not Build command - .net-core

Question

I am struggling to understand the differences between dotnet run and dotnet build commands, in regards to constants defined in the .csproj file
example.csproj
<Project Sdk="Microsoft.NET.Sdk">
<PropertyGroup>
<OutputType>Exe</OutputType>
<TargetFramework>netcoreapp3.1</TargetFramework>
</PropertyGroup>
<PropertyGroup Condition="'$(Configuration)'=='Test'">
<DefineConstants>$(DefineConstants);INCLUDE_TESTS</DefineConstants>
</PropertyGroup>
</Project>
program.cs
using System;
namespace define_example
{
class Program
{
static void Main(string[] args)
{
#if INCLUDE_TESTS
Console.WriteLine("It worked!");
#endif
Console.WriteLine("Hello World!");
}
}
}
If I run dotnet run -c Test, -> It worked! Hello World!
If I run dotnet build -c Test, then dotnet run {buildpath} -> Hello World!
Why does the defined constant go missing when I run the build command?

Answer 1

dotnet build and dotnet run are both commands that act in relation to a csproj file, not a dll or executable.
dotnet build will call the dotnet restore command and then build the project.
dotnet run will call the dotnet build command (this is important to note with respect to your issue) and then run the resulting dll or exe.
What you're currently telling dotnet to do by calling dotnet build -c Test and dotnet run is:
clean, restore, build (using the "Test" configuration)
clean, restore, build, run (with no configuration specified)
The second clean will cleanup the files produced by the first build and then you will build again as part of the run command, but without the Test configuration specified, causing the observed behaviour.
You could change dotnet run to dotnet run --no-build to tell it to assume a build has already been completed and to just assume that the files that would have been created are present and correct.
Alternatively, given that your project has an OutputType of Exe, you could just invoke the executable that is produced by the build command in the regular way as the second step. i.e. using the command example.exe.
If your OutputType was unspecified, or the default (Library) then this would produce a dll rather than an exe, and you could run it using the dotnet command: dotnet example.dll.
Useful reading on the various commands and also common properties:
dotnet command
dotnet build command
dotnet run command
Common MSBuild project properties