Working with the using directive in C

Microsoft documentation

The using directive allows you to use types defined in a namespace without specifying the fully qualified namespace of that type. In its basic form, the using directive imports all the types from a single namespace, as shown in the following example:

using System.Text;

You can apply two modifiers to a using directive:

The global modifier has the same effect as adding the same using directive to every source file in your project. This modifier was introduced in C# 10.
The static modifier imports the static members and nested types from a single type rather than importing all the types in a namespace.

You can combine both modifiers to import the static members from a type in all source files in your project.

You can also create an alias for a namespace or a type with a using alias directive.

using Project = PC.MyCompany.Project;

MSBuild project file (Using)

Another way to use the using directive is in a project file as shown below, taken from this project.

<ItemGroup>
    <ProjectReference Include="..\SqlServerLibrary\SqlServerLibrary.csproj" />
</ItemGroup>

You can also use the Using item to define global using <alias> and using static <type> directives.

Now knowing this, we can use this with an alias, in this case because there is a class named DataTable and we also want to use System.Data.Common.DataTable throughout several classes in the current project. We could use an Using directive in each class but using the following to use DataTable class from the SqlServerLibrary project simply prefix with SLD.

<ItemGroup>
    <Using Include="SqlServerLibrary.DataHelpers" Alias="SLD" />
</ItemGroup>

Usage

SLM.DataTable dataTable1 = new SLM.DataTable

Then to use a System.Data, DataTable add a Using directive.

using System.Data;

Sample code

SLM.DataTable dataTable1 = new SLM.DataTable
{
    CaseSensitive = true,
    Stash = "secret stuff"
};

Console.WriteLine($"{dataTable1.Stash, -16}{dataTable1.CaseSensitive}");

/*
* System.Data.DataTable
*/
DataTable dataTable2 = new DataTable();
dataTable2.CaseSensitive = true;
Console.WriteLine(dataTable2.CaseSensitive);

Suppose a developer encounters issues like above with DataTable in more than one project.

Create a new text file in the project named Directory.Build.props. Then setup up as follows which works the same as having the Using Include in the project file.

<Project>
    <ItemGroup>
        <Using Include="SqlServerLibrary.DataHelpers" Alias="SLD" />
    </ItemGroup>
</Project>

Copy the above file to other projects which can now use the alias SLD.

If the above technique is not for you than add a conventional Using directive with an alias to the top of a class.

using SLD = SqlServerLibrary.DataHelpers;

static modifier

From Microsoft docs

The using static directive names a type whose static members and nested types you can access without specifying a type name. Its syntax is:

using static <fully-qualified-type-name>;

The <fully-qualified-type-name> is the name of the type whose static members and nested types can be referenced without specifying a type name. If you don't provide a fully qualified type name (the full namespace name along with the type name), C# generates compiler error CS0246: "The type or namespace name 'type/namespace' couldn't be found (are you missing a using directive or an assembly reference?)".

The using static directive applies to any type that has static members (or nested types), even if it also has instance members. However, instance members can only be invoked through the type instance.

There are two obvious uses

The Console class: using static System.Console; which means instead of typing Console.ReadLine simply type ReadLine
System.DateTime, rather than typing DateTime.Now with using static System.DateTime; type Now.

Another example which keeps code clean and easy to understand.

using static System.Globalization.DateTimeFormatInfo;

namespace MSBuildUsingExampleApp.Classes;
internal class DateTimeHelpers
{
    public static List<string> MonthNames()
        => CurrentInfo!.MonthNames[..^1].ToList();
}

The downsides to using static is when code becomes difficult to read/understand were methods comes from and if that becomes the case revert back to a standard using directive or with an alias e.g.

using C = System.Console;
...
C.WriteLine(...

Or we can use the following in a project file to eliminate the using directive for System.Console.

<ItemGroup>
    <Using Include="System.Console" Static="True" />
</ItemGroup>

Project code

There are plenty of comments
SqlServerLibrary checks the existence of a SQL-Server database for SQLEXPRESS which is not installed will raise an exception.

Source code

Clone the following GitHub repository and work with the project MSBuildUsingExampleApp.

Coded with Microsoft Visual Studio 2022, .NET Core 7.

Working with the using directive in C#