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How to use a "flat" namespace in whole C# project

I want to use the same namespace for my whole c# class library. But the default behavior for new files is to use a namespace like [Base namespace].[Folder 1].[Folder 2].
Is there a simple way or plugin for visual studio 2019 to make all new files use the same namespace? So just [Base namespace] whatever the folder?
But just for a single project, I don't want to change the global file templates.

Although all concerns mentioned in comments, there's simple way to achieve this. Just go to project properties and define default namespace:
Now every added class has this namespace defined:
using System;
using System.Collections.Generic;
using System.Text;
namespace MyGlobalNamespae
{
class Class1
{
}
}

I have found a workaround I'm pleased with:
I installed this nice extension Visual Studio Namespace Fixer. It allows you to define a template for your namespaces and then can apply it to all .cs, .xaml, and other files in your project.
Under Options -> Namespace Fixer options -> Use default project namespace I defined the format {projectRootNamespace} for Namespace format.
Not I just have to select my files and folders in the projects base folder (in Visual Studio, not in the windows explorer), rightclick and select Adjust namespaces and all files in all subfolders will change their namespace to the projects root namespace :-)
Awesome if you create and use a lot of class and other files in your NuGet lib.

How to set a namespace for custom control

I have been implementing this solution explained here to make an outlinedTextbox.
I have created a test project and added directly in the main namespace and it works.
Now I want to add it to a library (HelperLib) and want to use it in whatever program of mine I want. For example here the program is called pcdLoggerS2.
but when I add it to my xaml it says that
so it's a matter of namespaces.
Therefore I have add this
xmlns:local="clr-HelperLib"
in my definition of my Base:WindowViewBase but nothing has changed but it's really in that namespace!
--ADD FOR DAVID---

You need to make sure that you've added the HelperLib project as a reference to your test project.
Open PcdLogger and right click on References, and add that project as a reference. Until you do this, the XAML in your test project will not be able to find the correct assembly.
Additionally, when you reference a namespace, from another assembly, you need to add that information to the namespace declaration in your XAML.
(See: https://msdn.microsoft.com/en-us/library/bb514546%28v=vs.90%29.aspx)
As an example:
(I would suggest leaving local for your local namespaces)
xmlns:helper="clr-namespace:HelperLib;assembly=HelperLib"
EDIT:
Additionally, Visual Studio's intellisense is fond of telling you that the namespace, or class, does not exist until you have built the project. You may have to rebuild and/or close/reopen the xaml file for intellisense to cooperate again.

check Namespace of you class.
HelperLib its name of a class, but not namespace.

Try to write this:
xmlns:local="clr-namespace:HelperLib"

C# using others code

Iv'e downloaded a C# interval tree collection class class from here http://intervaltree.codeplex.com/SourceControl/list/changesets -> Right hand side -> Download.
However I can't open the whole project on my Microsoft Visual C# 2010 Express (that also runs C# XNA) because
Solution folders are not supported in this version of the application
Also I just want the class to use separately in my own seprate project.
I tried to copy the three important seeming files Interval.cs, IntervalNode.cs and IntervalTree.cs into my project but this generated the compile error
There are no importers which handle this file type
I've also tried to copy and paste the contents of the three files into my project, encapsulating them into there own namespace as well as there was a lot of code. I had to rearange some of the usings a little but have run into the problem that possibly it wants PowerCollections .dll and .pcb files as using Wintellect.PowerCollections; causes
The type or namespace name 'Wintellect' could not be found (are you missing a using directive or an assembly reference?)
I'm not sure how to continue or if I'm doing the right thing at all in how to get this class to work.

Add the library to your solution
Copy the IntervalTreeLib directory into your solution directory. Then, right-click your solution, and add existing project. Point it at IntervalTreeLib.csproj in IntervalTreeLib, and click Open. That should add the IntervalTreeLib project to your solution.
Add a reference to the library in your project
Then, in your project, add a reference to the IntervalTreeLib proejct:
- Right click the References folder, and Add Reference. Click the Projects tab, and select IntervalTreeLib.
Use the classes in your code
To use classes from the library in your source then, you need to either add:
using IntervalTreeLib;
void Foo() {
IntervalTree<int, int> tree = new ...
}
Or, refer to them by their full name:
IntervalTreeLib.IntervalTree<int, int> tree = new ...

Open the IntervalTreeLib.csproj file if you want to be able to open the project in it's entirety (or in your current solution add an existing project (you can right-click on the solution) and select the IntervalTreeLib.csproj). If you are trying to grab just the code file in your project, ensure you also grab the PowerCollections.dll file (I see it is in the same folder as the code files) or your code will not compile (as you have discovered). You'll need to add a reference to it and include the needed using statement at the top of the code files making use of this library (or use fully qualified name with the namespace).
using IntervalTreeLib;
or
var myObj = new IntervalTreeLib.[WhateverClass](...);
Also, make sure you read the license.txt file. You may need to include it if you are using the code. Give credit where it is due.
UPDATE:
If the test project is causing you problems, just open the library project. Ideally you could just open that and compile it, adding the output DLL files that are generated directly into your solution. This is ideal unless you are planning on changing the library source code itself.

Add the library to the references of the project you want to use it.

Since discussing that you are able to build Intervallib.dll, we will discuss about how you should the dll in your project.
Now in your proj, right click on the references part and add the dll intervallib.dll to your references. In your game.cs file, have the reference to the namespace as -- using IntervalTreeLib;
then you should actually copy the dll powercollections.dll to the bin directory of proj directory also.
you should copy this dll because there is an indirect link to the dll as it is used in IntervalTreeLib.dll
following these steps, I was able to execute this project.

What's the best way to do a bulk namespace rename on a large c# application?

First, a little background.
Currently namespaces and assemblies in our codebase (~60 assemblies, thousands of classes) looks like
WidgetCompany.Department.Something
We have now been spun off such that we are selling the software that drives a WidgetCompany, so we 'd like to rename the namespaces & assemblies
NewCompany.Something
Under normal circumstances I'd probably just stick with the old namespace, but the problem is our customers don't want to see the name of one of their competitors anywhere in the application. In the off chance that they see a stack trace, assembly properties etc, it shouldn't show up. It's not meant to hide our associates or do anything sinister, we just need to make sure that people know we are a separate entity, separate management, and they don't need to worry about data being shared etc.
Now the question. What is the best way to perform this type of all encompassing rename?
The following would need to change:
Namespace for (almost) every class in
the application
Every using statement in the application which references the old names
Folder structure for each project
References between projects which rely on changed folder structure
.Sln files which reference the changed folder structure
Any references to
those classes which are fully
qualified (should be few and far
between)
Any references to those
classes in xml config files (config
sections etc)
AssemblyInfo.cs files for every assembly
AssemblyName in every .csproj file
Am I stuck with the find-replace-pray strategy or is there something better?

Right click on your current namespace and select Refactor -> Rename and change the name in the pop up that comes up after a while. Enter your new name and click ok.
If you have multiple depths to your namespace, then Visual Studio won't let you type a dot. However, if you copy and paste a dot, despite a warning, it will do the business.
To completely change to the new name, you will likely need to make additional changes manually. You can find where by performing a project search (ctrl+shift+f) for other references to the name in the Project, Solution, or other supporting files from a separate text editor like VS Code. Finally, folders may need to be changed manually as well.

ReSharper. Get version 5.1 from JetBrains for free for 30 days (more than enough time to do your renaming). Put the cursor on the namespace you want to change and hit Ctrl-R twice (ReSharper Rename). It'll work with a namespace any number of levels deep, and converts any usage of that namespace to the new one. However, you will have to change every unique namespace in your solution (unless you just go with Find/Replace)

Visual Studio 2019 Community Edition supports this as described here. It works for any hierarchy (with dots, root namespace changes etc) and correctly updates all dependencies.
Place your cursor in the class name
Press Ctrl+. to trigger the Quick Actions and Refactorings menu
Select Move to namespace
In the dialog box that opens, select the target namespace you'd like to move the type to

If you have ReSharper:
Right click project, Properties. Change Default namespace to desired
namespace.
Right click project, Refactor -> Adjust Namespace to update all
the namespaces to use the default namespace
Then just let it do its magic.

Firstly I would try Refactor->Rename option. But as mentioned in comment for another answers it doesn't work that good (Or I haven't found how to make it working). So I prefer using following scenario, especially if you want to add some addiotional namespace.
1) Rename your root namespace (WidgetCompany) to something like NAMESPACE_TO_BE_REPLACED using Refactor-Rename
2) Replace NAMESPACE_TO_BE_REPLACED with your final namespace (NewCompany.WidgetSoftware) using find-n-replace dialog
And do not forget to rename projects, default namespaces, etc.

Open a random class in [WidgetCompany.Department.Something]
Edit->Find and Replace->Replace in Files
Type "WidgetCompany.Department.Something" in FindWhat area
Type "NewCompany.Something" in Replace with area
Select Current Project in Look in area.
Replace All

Alternative solution if you've already partially renamed:
Open find and replace (shortcut ctrl + h)
Make sure regex is selected (icon like .*)
Paste the full name of your current namespace, for example YourSolution.YourProject
Paste this regex YourSolution.Your[partiallyRenamedNamespaceCharacters].*
Make sure Entire Solution is selected not Current Document
Run

For Visual Studio 2022 this has finally been solved:
Right-click on the project or solution (not the directory or file)
Select "Sync Namespaces"
The namespaces in your entire project should now reflect the folder structure.

Refactor namespace in Visual Studio [duplicate]

I'm doing some architectural cleanup that involves moving a bunch of classes into different projects and/or namespaces. Currently I'm moving the files by hand, building, and then manually adding using Foo statements as needed to resolve compilation errors. Anyone know of a smarter way of doing this? (We're a CodeRush and Refactor! shop, but I'd be interested to hear if Resharper has support for this)

Visual Studio 2019 provides at least 2 built-in options:
'Move to namespace...' refactoring can be triggered on any class, and VS will prompt for the target namespace.
'Change namespace to...' refactoring is provided for when the current file namespace doesn't match with the folder structure.
This can be used to move individual classes to a different namespace by:
creating the desired folder structure
moving the file
applying the mentioned refactoring (CTRL+. with the cursor over the namespace)
These operation ensures that all references are updated accordingly.

Visual Studio 2010 has the possibility to rename a namespace. Place the cursor over the namespace name and press F2. Or simply rename it in the code and press Shift+Alt+F10, Enter after seeing the red squiggle appear.
Reharper can also rename namespaces. Quote:
The Rename Namespace refactoring
allows users to rename a specific
namespace and automatically correct
all references to the namespace in the
code. The following usages are
renamed:
Namespace statements
Using directives
Qualified names of types

As mentioned in the comments, this answer is now outdated. Please see the up-to-date answer below
Resharper is the only tool I am aware of what has this ability. There is also a lot of other functionality that it has that is missing in CodeRush and Refactor!

This answer applies to at least Visual Studio 2013 and 2015 with no resharper required
Move class files to new folder
Open 'Find and replace'
Select 'Replace in Files'
Type the original namespace definition in the 'Find what' field eg. MyCorp.AppStuff.Api
Type the new namespace definition in the 'Replace with' field eg. MyCorp.AppStuff.Api.Extensions
Select the new folder using the 'Look in' field's browse button ..., or type the folder path
Press the Replace All button

There are partial solutions for VS 2015 & VS 2017 without Resharper using free extensions.
One extension which I like today (end of 2017) is the Fix Namespace VS Extension:
https://marketplace.visualstudio.com/items?itemName=vs-publisher-599079.FixNamespace#overview
It analyses the folder structure of your solution and offers namespace refactoring using that. Unfortunately it isn't perfect: It doesn't track dependencies that well, but solved the lion's share of the work for me.

With Resharper: CTRL+R+O
Then press the down arrow key twice to select Move Type To Another Namespace.

Since the answer above was provided (I'm guessing) this feature has been added to CodeRush. Just place the carat on the Type to be moved and you'll see a Move Type to Namespace option on the Refactor! context menu. This will move the type to the new namespace and update references. You may still want to move the file to a solution folder that matches the name of the namespace though.

It's not the best outcome but can be done without plugins or tools, only with Visual Studio. Find and replace in Entire Solution, Match case, Match whole word.
Find what: class name, Replace with: New.Namespace.ClassName (fully qualified class name).
If you have 100+ references of the moved class and other classes in old namespace what are not moved this is the only foolproof and free solution I found. The only case when it leads to errors is when you have same class name in other namespace.

If you cannot, or do not want to use Re$harper, Notepad++ is your friend:
Make sure you don't have usaved changes inside Visual Studio for the files you need to move to the new namespace
Open all the files that contain the namespace that needs to be changed in Notepad++
Open Find & Replace (CTRL + H)
Fill the Find what and Replace with fields
Press Replace All in All Opened Documents
Save all changes in all documents (CTRL + SHIFT + S)
Switch to Visual Studio and reload all the documents (Yes to all at the prompt)
DONE