本文翻譯自：The type or namespace name could not be found [duplicate]
This question already has an answer here: 這個問題在這裏已有答案：
I have a C# solution with several projects in Visual Studio 2010 . 我有一個C#解決方案，在Visual Studio 2010有幾個項目。 One is a test project (I'll call it " PrjTest "), the other is a Windows Forms Application project (I'll call it " PrjForm "). 一個是測試項目（我將其稱爲“ PrjTest ”），另一個是Windows Forms Application項目（我稱之爲“ PrjForm ”）。 There is also a third project referenced by PrjForm, which it is able to reference and use successfully. 還有一個由PrjForm引用的第三個項目，它能夠成功引用和使用。
PrjForm references PrjTest , and PrjForm has a class with a using statement: PrjForm引用PrjTest ， PrjForm有一個帶有using語句的類：
using PrjTest;
- Reference has been correctly added 參考已正確添加
-
usingstatement is correctly in placeusing語句正確到位 - Spelling is correct 拼寫是正確的
- PrjTest builds successfully PrjTest構建成功
- PrjForm almost builds, but breaks on the
using PrjTest;PrjForm幾乎構建，但打破了using PrjTest;line with the error: 與錯誤一致：
The type or namespace name 'PrjTest' could not be found (are you missing a using directive or an assembly reference?) 找不到類型或命名空間名稱'PrjTest'（您是否缺少using指令或程序集引用？）
I've tried the following to resolve this: 我嘗試過以下方法來解決這個問題：
- Removed Resharper (since Resharper had no trouble recognizing the referenced project, I thought it might be worth a shot) 刪除了Resharper（因爲Resharper在識別引用的項目時沒有遇到任何麻煩，我認爲它可能值得一試）
- Removed and re-added the reference and using statement 刪除並重新添加了引用和using語句
- Recreated PrjForm from scratch 從頭開始重新創建PrjForm
- PrjForm currently resides inside the PrjTest folder, I tried moving it to an outside folder PrjForm目前駐留在PrjTest文件夾中，我嘗試將其移動到外部文件夾
- Loaded the solution on a different computer with a fresh copy of
VS 2010使用VS 2010的新副本在另一臺計算機上加載解決方案
I have done my homework and spent far too long looking for an answer online, none of the solutions has helped yet. 我做完了我的功課，花了很長時間在線尋找答案，但沒有一個解決方案有幫助。
What else could I try? 我還能嘗試什麼？
#1樓
參考：https://stackoom.com/question/JzaU/找不到類型或命名空間名稱-重複
#2樓
just changed Application's target framework to ".Net Framework 4". 剛剛將Application的目標框架更改爲“.Net Framework 4”。
And error got Disappeared. 錯誤消失了。
good luck; 祝好運; :D ：d
#3樓
I had the same issue. 我遇到過同樣的問題。 The target frameworks were fine for me. 目標框架對我來說很好。 Still it was not working. 它仍然無法正常工作。 I installed VS2010 sp1, and did a "Rebuild" on the PrjTest. 我安裝了VS2010 sp1，並在PrjTest上進行了“Rebuild”。 Then it started working for me. 然後它開始爲我工作。
#4樓
Changing the framework to 將框架更改爲
.NET Framework 4 Client Profile
did the job for me. 爲我做了這份工作。
#5樓
For COM/ActiveX references, VS 2012 will show this error right on using statement. 對於COM / ActiveX引用，VS 2012將在using語句上顯示此錯誤。 Which is quite funny, since it's saying that may be you are missing a using statement. 這很有趣，因爲它說你可能錯過了一個使用聲明。
To solve this: register the actual COM/ActiveX dll even if it's in the neighbor project, and add a reference through COM channel, not project channel. 要解決此問題：即使它位於鄰居項目中，也要註冊實際的COM / ActiveX dll，並通過COM通道而不是項目通道添加引用。 It will add Interop.ProjectName instead of ProjectName as a reference and this solves this strange bug. 它將添加Interop.ProjectName而不是ProjectName作爲參考，這解決了這個奇怪的錯誤。
#6樓
In my case I had: 在我的情況下，我有：
Referenced DLL : .NET 4.5 引用的DLL： .NET 4.5
Project : .NET 4.0 項目： .NET 4.0
Because of the above mismatch, the 4.0 project couldn't see inside the namespace of the 4.5 .DLL. 由於上述不匹配，4.0項目無法在4.5 .DLL的命名空間內看到。 I recompiled the .DLL to target .NET 4.0 and I was fine. 我重新編譯.DLL以目標.NET 4.0，我很好。