C#: Difference between using System.Text and System.Text.RegularExpressions

In an ASP.NET C# application, I noticed in order to use Regex and StringBuilder, I had to put both

using System.Text;
using System.Text.RegularExpressions;

From looking at this plainly, I would think using System.Text might include RegularExpressions, but both are necessary. What is the difference between those two?

Answers:

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Method 1

They are different namespaces with different classes. Including the top level namespace (System.Text) doesn’t mean that namespace below (like RegularExpression) will be added.

StringBuilder is from System.Text so that is why you need using System.Text; and Regex is from System.Text.RegularExpressions and that is why you need that as well.

See: using directive from C# Specifications section 9.4.

The scope of a using-directive extends over the
namespace-member-declarations of its immediately containing
compilation unit or namespace body. The scope of a using-directive
specifically does not include its peer using-directives. Thus, peer
using-directives do not affect each other, and the order in which they
are written is insignificant.

Method 2

• Regex is defined in System.Text.RegularExpressions
• StringBuilder is defined in System.Text

You need to include both namespaces for these two types to be in scope. Including a namespace A.B doesn’t automatically include a contained namespace A.B.C. If this were the case, nested namespaces would be of limited use.

Method 3

From MSDN:

The System.Text namespace contains classes that represent ASCII and
Unicode character encodings; abstract base classes for converting
blocks of characters to and from blocks of bytes; and a helper class
that manipulates and formats String objects without creating
intermediate instances of String.

And:

The System.Text.RegularExpressions namespace contains classes that
provide access to the .NET Framework regular expression engine. The
namespace provides regular expression functionality that may be used
from any platform or language that runs within the Microsoft .NET
Framework. In addition to the types contained in this namespace, the
System.Configuration.RegexStringValidator class enables you to
determine whether a particular string conforms to a regular expression
pattern.

StringBuilder lives in the System.Text namespace and Regex lives in System.Text.RegularExpressions. These namespaces are just logical separators, and including a parent namespace doesn’t automatically include children, and vice versa.

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