The Project File

Every project has one project file sitting next to the code, named after the project and ending in .csproj. It is the instruction sheet for the build: which .NET version to compile for, which outside libraries to use, which compiler options to switch on.

It is XML because the build engine, MSBuild, reads XML. XML is a text format that nests tagged elements, so <TargetFramework>net10.0</TargetFramework> is one setting named TargetFramework with the value net10.0. The file is short because the opening <Project Sdk="Microsoft.NET.Sdk"> brings in a pile of defaults. One of them compiles every .cs file in the folder and below, so files are never listed by hand.

The settings in a new console project

OutputType set to Exe means the build produces something runnable; leave it out and you get a library, which has no entry point and cannot be started. TargetFramework set to net10.0 means compile against .NET 10's libraries and require that runtime. Nullable set to enable turns on the compiler's null checking, which warns you when a value that might be absent is used as if it never could be. ImplicitUsings set to enable imports seven namespaces into every file for you: System, System.Collections.Generic, System.IO, System.Linq, System.Net.Http, System.Threading and System.Threading.Tasks.

Why net10.0 appears only here

The target framework is a build setting, so no .cs file ever names it. It is also not the SDK version: dotnet --version reports the SDK, a different number, and a newer SDK still builds a net10.0 project unchanged.

// ImplicitUsings is enabled in the project file, so System.IO is already
// imported here. Without it, this file would need: using System.IO;
Console.WriteLine(Path.GetFileName("reports/january.csv"));

// Nullable is enabled, so the question mark is how you say out loud that
// this variable is allowed to hold nothing at all.
string? missing = null;
Console.WriteLine(missing is null);

// Output:
// january.csv
// True

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net10.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

</Project>

// With <ImplicitUsings>disable</ImplicitUsings> in the project file, the
// imports have to be written out. These using directives are exactly what
// the setting was adding for you, so the two versions are the same program.
using System;
using System.IO;

Console.WriteLine(Path.GetFileName("reports/january.csv"));

// Output:
// january.csv

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net10.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
    <!-- Refuse to build while any warning remains. -->
    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
  </PropertyGroup>

  <ItemGroup>
    <!-- Import one more namespace in every file of this project. -->
    <Using Include="System.Text.Json" />
    <!-- Stop importing one the SDK adds by default. -->
    <Using Remove="System.Net.Http" />
  </ItemGroup>

</Project>