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.NET/C# assemblies and namespaces

Posted by jpluimers on 2013/07/23

Ever since I started .NET programming after .NET Beta 1 Arrived in 2001, I found that many people struggle with the relation between assemblies and namespaces.

So I was glad that I posted this answer about 2.5 years ago on StackOverflow. Below is the slightly edited form:

People are easily confused by the namespace/assembly thing, as it decouples the concept of where your code is physically located (the assembly) and how you reference it:

  • logically reference is by using the namespace
  • physical reference is by referencing the assembly

I usually explain the relation using the word contribute:

  1. An assembly can contribute to multiple namespaces.
    For instance, the System.Data.dll assembly contributes to namespaces like System.Data (e.g. the class System.Data.DataTable) and Microsoft.SqlServer.Server (e.g. the class Microsoft.SqlServer.Server.SqlContext).
  2. Multiple assemblies can contribute to a single namespace.
    For instance both the System.Data.dll assembly and the System.Xml.dll assembly contribute to the System.Xml namespace.
    Which means that if you use the System.Xml.XmlDataDocument class from your project, you need to reference the System.Data.dll assembly.
    And if you use the System.Xml.XmlDocument class, you need to reference the System.Xml.dll from your project.

(the above examples are .NET 4.0, but likely hold for previous .NET versions as well).

Danny Thorpe explained the concept of namespace and internal really well, so I won’t go into detail about those.

Ever since I started .NET courses 10 years ago, I draw a table explaining assemblies and namespaces like this:

Assemblies contributing to namespaces
Assembly Namespaces it contributes to
System.Data Microsoft.SQLServer.Server System.Xml
↑ Example classes
System.Data.dll DataTable SqlContext XmlDataDocument
System.Xml.dll XmlDocument


via: C# assemblies, whats in an assembly? – Stack Overflow.

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