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How do I enable prediction when writing C# codes in Visual Studio Code? [duplicate]

I just downloaded Visual Studio Code and my Intellisense is not automatically working.
The two settings that control this seem to be set correctly:
"editor.quickSuggestions": true,
"editor.suggestOnTriggerCharacters": true,
I do get an Intellisense menu when i start typing and press "CTRL + Space", but this gives me a list of everything and not things specifically for my object.
What am i doing wrong?

If you have a workspace and for some reason have multiple folders you may need to 'help' omnisharp a bit. I initially had a big project and added a solution for it further on - ending up with TWO workspace folders (one to a startup project and one to the solution). After composing that setup I experienced only the first project to have intellisense working.
Solution to get intellisense working was to make sure omnisharp worked its way from the solution instead of the project:
Ctrl + Shift + p
Write "OmniSharp: Select Project" and press Enter.
Choose the solution workspace entry.
Inspiration gotten from 'swaner':
https://github.com/OmniSharp/omnisharp-vscode/issues/1889

Visual Studio Code's more advanced editing features (IntelliSense, refactoring tools, etc.) only work in C# files if you have a project.json file or *.sln file that VSCode is aware of. Open the folder (i.e. open the File menu and click Open Folder...) with the *.sln file or project.json and VSCode will attempt to find all project/solution files in the folder. If there are multiple projects, you may need to select one from the projects button on the right side of the status bar (bottom of the window).
From the VSCode website:
Selecting a project.json-file is opening a DNX-project and VSCode will load that project plus the referenced projects
Selecting a *.sln-file is opening a MSBuild-project. It will load the referenced *.csproj-projects and sibling or descendant project.json-files but no other project files that are referenced from the solution file.
Selecting a folder will make VSCode scan for *.sln and project.json files and VSCode will attempt to load them all.

If you are start your project with c#, then some time you haven't download extension.
Process by written
Ctrl + Shift + p.
Write "OmniSharp: Select Project" and press Enter.
Choose the solution workspace entry.
Then enable the c# extension for "OmniSharp : Project".
Process by image

Downgrading to 1.23.9 of the C# VS Code extension fixed the intellisense for me. So if nothing else is working for you, perhaps try installing an older version of the extension (doesn't necessarily have to be that exact one I mentioned). You can do this by selecting this option in VS Code:

As of the 0.5 version I have found that I need to close and re-open my working files to get some new intellisense information to start working for my own files. This usually happens after I have a file open that needs to use type information from another file I just created. Until I close and re-open them (within the editor) the intellisense doesn't always work.
Hopefully this is something that'll get fixed as the app matures.

Inorder to make the intelligence working, the Visual Studio Code extension OmniSharp should be there along with Mono. When you open a project/solution folder, the OmniSharp gets the project dependencies from either project.json or from the dotnet solution file (if the project/solution created with other versions of visual studio).
So, when you look at the omnisharp's output window after immediately opening the project/ solution folder, you may see the following lines
Starting OmniSharp server at 2017-9-18 09:26:44
Target: /Users/{username}/Source/{Your Solution Folder}/{Your Solution
file}.sln
OmniSharp server started wth Mono
Path: /Users/{username}/.vscode/extensions/ms-vscode.csharp-1.12.1/.omnisharp/run
PID: 5808
post that, you may see couple of lines coming up like below,
[info]: OmniSharp.MSBuild.MSBuildProjectSystem
Update project: {Your Project1 Name}
[info]: OmniSharp.MSBuild.MSBuildProjectSystem
Update project: {Your Project2 Name}
....
they mean that your projects are getting recognised by OmniSharp Extension.
If you get any error message informing you about timeout, please get into the settings of the Visual Studio Code, and add a configuration override like the one below:
"omnisharp.projectLoadTimeout": 200
I know it is too long to wait for 200 seconds. But don't worry, this won't stop you anymore working with project files. But, remember that the IntelliSense will be automatically available once all the projects in the folder are successfully loaded.
Feel free to extend the Timeout setting since it will help you getting intellisense even-though you are not getting it immediately.
Here is another link with the same solution: https://github.com/OmniSharp/omnisharp-vscode/issues/1585
Hope my information helps you! Enjoy your VSCode!

This case was for an existing project that was working fine before. Opened via the vs code "recent" history.
For dotnet core. I opened my project's *.csprog file, made no changes, and saved it via the vs code editor.
As soon as I saved, all of the intellisense stuff starting working again in my other files within that project.

Another troubleshooting strategy to try if none of the above works out is uninstalling Mono, which is unnecessary anyways if you're using .NET Core. I also experienced a sudden disappearance of Intellisense after pulling changes to one of our enterprise repos that included an upgrade from ASP.NET Core 2.0 to 2.1. Strangely, at the same time, Intellisense was working fine in another C# project repo running ASP.NET Core 2.2.
For the project in question, I was puzzled to see successful logs for each reboot of the Omnisharp server as well as perfectly clean builds and runs. The puzzlement increased as I compared these Omnisharp logs to those from the 2.2 project and found them effectively identical.
Upon uninstalling Mono, the one detail that changed is how Omnisharp started. Now it boots up from a shell script located at ~/.vscode/extensions/ms-vscode.csharp-1.19.0/.omnisharp/1.32.18/run. Previously, Mono itself was booting the server from the executable at ~/.vscode/extensions/ms-vscode.csharp-1.19.0/.omnisharp/1.32.18/omnisharp/Omnisharp.exe.
Other troubleshooting I attempted in this instance though to no avail:
dotnet clean / Deleting bin and obj directories
Updating launch.json to point at the proper /bin/Debug/netcoreapp2.1 build
directory
Wiping my local nuget cache
Restarting Omnisharp
Uninstalling and reinstalling VS Code, as well as bumping back a version from 1.33.1 as the breakage coincided oddly with the March 2019 update
Uninstalling and reinstalling the C# VS Code extension, as well as bumping back a version from 1.19.0 given other users' reports of said version interfering with Intellisense for certain projects*
Adding a global.json file at project root to override default utilization of latest .NET Core SDK (mine was 2.2.105) and run instead using 2.1.302 in concert with the project's version
Wiping/rebuilding the project .sln file
Specs: VS Code 1.33.1, C# Extension 1.19.0, MacOS High Sierra 10.13.6.
*NB: As it turns out, in another .NET Core 2.1 project with the same issues in VS Code as described above, uninstalling Mono alone did not fix things. I also did wind up needing to bump back the C# extension to v. 1.18.0 to recover Intellisense. Weird.

The issue I had was OmniSharp was an older version. I set the flag to update to latest version in Settings.json file. This ensures the extension is always the latest version.
"omnisharp.path": "latest"
And restarted VS code. That fixed it for me.

Simply reinstall the extension and restart the VSCode/PC.

I use Task Manager and kill the Visual Studio Code process, then restart Visual Studio Code, the intellisense shows and fixed.

I've had some good experiences in C# with this extension so far:
https://marketplace.visualstudio.com/items?itemName=jchannon.csharpextensions
This extension traverses up the folder tree to find the project.json or *.csproj and uses that as the parent folder to determine namespaces.

Try "Install Extension" from command Pallete - probably if C# intellisense is not there:
https://code.visualstudio.com/docs/customization/colorizer

if intellisense in not working for react js or javascript or node js this solution will help for windows
uninistall type script by typing npm uninstall -g typescript
install specific version of type script that is 3.3 by typing npm install -g typescript#3.3
add the typescript location in vscode by navigating to file > Preferences > settings > here search for edit in setting.json then add this path "typescript.tsdk": "/Users/yourusename/AppData/Roaming/npm/node_modules/typescript/lib"
settings.json should look someting like this
{
"editor.suggestSelection": "first",
"vsintellicode.modify.editor.suggestSelection":"automaticallyOverrodeDefaultValue",
"typescript.tsdk": "/Users/yourusername/AppData/Roaming/npm/node_modules/typescript/lib"
}

I was able to fix this by changing the Api Compatibility Level from .Net Standard 2.0 to .NT 4.x. You can find this setting on Project Settings / Player.
After that, intellisense started working again.

This is because of C# extension issue.
Reinstall the extension will work.
https://code.visualstudio.com/docs/editor/intellisense
https://marketplace.visualstudio.com/items?itemName=ms-dotnettools.csharp

I fixed this with a multiple project solution in .net5 / .net core 3.1 by
opening each project (not the sln folder) in VSCODE. This then prompts to add the resources/ .CSX? files to the project; enabling intellisense, per project. Finally close that window and reopen the solution folder and all works as expected.
See image
Add VSCode assets

Downgrading to 1.23.9 of C# for Visual Studio Code (powered by OmniSharp).
https://i.stack.imgur.com/VrdJl.png

After trying several things I looked at the OmniSharp logs only to realize that because I had two different sln files in my workspace. It had picked up the “other” one and thus wasn’t working. I deleted it, reopened vs code, and all is well again.

For me I had been working without internet for a long time. It had previously been working but then stopped. I closed+reopened VSCode with a stable wifi connection and the Output tab in VSCode popped up, displaying the the extensions were re-installing. After that it worked!

in my case , the extension wasn't enabled

I just had this happen (this being no code completion suggestions appearing). What resolved it for me was changing the VSCode Editor package in Unity to a previous version and then back to the current version.
For me, this was going from 1.2.0 to 1.1.3 and then back to 1.2.0. I believe removing and reinstalling 1.2.0 would have accomplished the same.
Window > Package Manager > Visual Studio Code Editor

I know this is probably the most obvious answer. But I had enough dealing with VS Code ridiculous bugs. You should be focusing on code, not the buggy IDE. Their documentation doesn't make it easy either to debug.
Steps for complete uninstallation:
Save your personalization files such as keybindings.json,
Just put it on GitHub or something so whenever you need have trouble with VS Code you simply just need to go to GitHub and copy-paste it.
Uninstall VS Code completely
When you uninstalled VS Code, it doesn't erase your extension files and some old settings. This is the cause of the re-installation ended up the same. Horrible move VS Code. For the list of the files that you must delete, you can check out this answer.
Go to their website and install the new one.
I hope this helps some people who are already had enough with the VS Code ridiculous bug.

In my case, the issue was not with VS Code settings. The error was due to a corrupted TEMP path configured in my PC's registry user profile environment settings.
I got this error from Help --> Toggle Developer Tools --> Console
The error was flagged in the console when I opened a new typescript file.
Unable to write file c:\Users\XXX\AppData\Local\Temp;C:\wamp64\bin\php\php5.6.25;C:\vufind-3.1.2\install.php\09cf49d8-af5b-42e9-8194-43f5b566be0f'
Obviously this path was corrupted. After updating the environment variables in registry settings, the VS code IntelliSense started working. Checking the console is a good start to find the root cause of this issue.

Earlier intellisense was working for Angular in VS-Code but for some reason (which I don't know) intellisense stopped working. I had extensions installed i.e. Angular Essentials and Angular Language Service etc. that were responsible for this intellisense and all that.
What I did?
I disabled the extensions, restarted VS Code and enabled them. It was all working fine as before.

If everything in VSCode is working ( No errors in output console, language server is working etc.) and built-in extensions are enabled. Still, if your IntelliSense is not working for normal .js files make sure variables are defined with a type identifier such as var d = new Date() or let d = new Date(). In my case, IntelliSense was not working for d = new Date() (No autocomplete for Date object 'd' APIs in this way) but started working fine when I specified a type before my variable names.

Download and install "Tool for Visual Studio 2019" as the C# extension under hood use the build tools: https://visualstudio.microsoft.com/downloads/

In my case, I had an extension installed (Explorer Exclude) to help me focus and hide certain files that I don't regularly work with within the Explorer.
I so happened to have disabled viewing *.sln, *.csproj and *.vsconfig files and folders, which apparently prevents VSCode from even knowing the file is there in the first place. Disabling these specific rules solved the problem instantly.
Took me ages to figure out... The more you know.

I solved it by uninstalling all SDK's (not sure if this is needed)
and installed 4.7.1 developer pack
win 10, vscode 1.63.2, unity 2020.3.25f1

I fixed this by installing .NET Framework 4.7.1 Developer Pack:
https://dotnet.microsoft.com/en-us/download/dotnet-framework/net471

OmniSharp intellisense not recognizing new class file

I have a dotnet core console app I generated from the cli via
dotnet new console -n EODG.VsCodeTester.Cli
Using the C# Extensions extension, I right click on the project folder in VSCode, and click "New C# Class".
The class/file opens just fine, but Omnisharp doesn't appear to recognize the file/class. From Program.cs, the class doesn't show up in the code completion, and the codelens in the new class doesn't show up.
The following is outputted from the OmniSharp log...
[info]: OmniSharp.Roslyn.CSharp.Services.Completion.CompletionService
Could not find document for file c:\Users\elrod\src\test\src\EODG.VsCodeTester.Cli\Cat.cs
Now, if I restart OmniSharp, it recognizes the new file just fine... but it's kind of a pain to restart omnisharp everytime I make a new class/interface or rename them. I've never seen this before, and I've been doing dotnet core development in vscode since v1.
I tried googling OmniSharp "Could not find document for file"... and it said No results found for OmniSharp "Could not find document for file".... so, that's nice.
Pertinent Details
OS: Windows 10 (fresh install... like less than 2 hours old)
VSCode version: 1.50.1
OmniSharp version: 1.23.5
Dotnet version: 3.1.403
I will be more than happy to provide any other details I can.
Thanks, y'all!

It looks like OmniSharp version 1.23.5 might be busted... I reverted to version 1.23.3, and things work as expected now. Possibly related to this issue on their git repo.
UPDATE: updating to v1.23.6 resolves this issue.

What worked for me on my mac, was that in settings, search for omnisharp, then look for the option that says Omnisharp: Use Global Mono and set it to always.

Steps to follow to make it work:
Find Assembly-CSharp.csproj.
Find the field under which your
old script names are present for compilation.
Add the new script name.
Save in Unity. Restart VS Code if required.
It should get solved.

Unable to start debugging. The startup project could not be launched. VS2015

I have started a new console project in VS2015. I only have this code :
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using SautinSoft;
namespace PdfParser
{
class Program
{
static void Main(string[] args)
{
string path = Environment.GetFolderPath(Environment.SpecialFolder.Desktop);
SautinSoft.PdfFocus f = new PdfFocus();
f.OpenPdf(#"path:\abc.pdf");
if (f.PageCount > 0)
f.ToExcel(#"path:\abc.xls");
}
}
}
When I try to run, I get this :
Unable to start debugging. The startup project could not be launched.
Verify debug settings for the startup project.
How can I fix this? Thanks.

After upgrading MSVC2015 to Update3, I had the same problem: instead of std::map in the Watch window display as a horrible std::_Tree<std::_Tmap_traits... etc., some plugins started to throw exceptions, some projects was not able to run in debug mode.
All was solved by deleting CurrentSettings.vssettings. It is usually located at Documents\Visual Studio 2015\Settings If not, go to `Tools -> Options -> Environment -> Import and Export Settings ' to find the location of this file.

Had this on a brand new scaffolded ASP.NET Core 1.0 web application.
Solved with restart of Visual Studio.

I was also stuck with this. I removed both bin & obj folders, did a rebuild, and then it launched.
A very meaningless error....

I had this problem when I tried to start (without) Debugging my Asp.Net MVC project
are you running Visual studio as Administrator
so just restart visual studio As Administrator

Additionally to "Unable to start debugging"
I also had errors "The operation could not be completed: Unspecified error" when I tried to rebuild.
Thanks to Visual Studio popup: "the operation could not be completed"
it was enough to close Visual Studio 2015 and then open again.

Make sure you're launching Visual Studio as Administrator.
Right click on the Visual Studio 2015 Shortcut
Select the Shortcut tab
Click on Advanced
Check the box to "Run as administrator" at all times(see below)
This has resolved the issue for me with the exact error message in question.
It is not ideal to run the VS in administrator mode at all times. Make sure you un-check the box once your issue has been resolved.

There is no need to delete everything in my case, I just opened the
"CurrentSettings.vssettings"
file and commented this property
<PropertyValue name="DefaultBehaviorForStartupProjects">1</PropertyValue>
and everything fixed.
Sample

Just delete the .vs hidden folder. This folder resides at the same location where the sln file is. Deleting this folder also refreshes intellisense cache and would fix any issues with that. Hope this helps.

VS 2019, two Core 3.1 projects (an MVC and an API), brand new projects created, no other changes to the solution or projects.
I kept getting the referenced error when trying to start both projects within the IDE, despite having checked all the things already mentioned in this SO.
However, I finally realized, I forgot to specify the "Action" after switching the solution to "Multiple startup projects" (they were both still set to "None"). Right-click solution in "Solution Explorer", choose "Properties" (at very bottom).
Such a ridiculously simple thing. Hope it helps someone else.
"Broken"
Working

I just had the same problem and solved it by deleting the .vs directory in the project directory.

Delete all relevant /obj and /bin folders. Run again.

Close the Visual Studio and re-open it again.

Just restart your visual studio and run as admin.

I had this with a Xamarin.iOS project in my Xamarin.Forms solution. I tried every suggestion here, without success.
The source of the error was a misconfigured .csproj of the project. I never edited it myself and not even touched the project properties. I use Visual Studio 2017 RC.
I came across the solution when I compared the project file with previous versions on Source Control.
Solution: Compare the project file with a previous version and try to granually revert changes until you eliminated the problem and know what the cause was.

The solution for me was to delete all *.user files that are normally generated for both solution and project files

I found this problem too. But after restart the Visual Studio Community 2015 as an administrator I found new problem :
And then I try to open another solution, close the another solution, and open the solution which the problem occur, then the problem solved.

Had the Same Issue with the existing application, I deleted the vs settings but still the issue remained,
Finally i just restarted the VS and it worked fine.
I think sometimes when we load the project some library doesn't get loaded with the solution so the best option before doing anything is simply restarting the VS.

After adding a project reference 'MyProjectReference' to my app, then removing it, I was also getting the same error :
Unable to start debugging . The startup project could not be launched.
Verify debug settings for the startup project.
Deleting configuration, running as admin, restarting VS, rebooting machine all had no effect.
After running my web app without debugging (Right-Click Project -> View In Browser (Google Chrome) I got a much more useful error:
Could not load file or assembly 'MyProjectReference' or one of its
dependencies. An attempt was made to load a program with an incorrect
format.
My Solution was then to re-add back to the solution, then remove it again. After that rebuild and debug worked.

How I fixed my issue, I closed project that had that error , created new project build new project then opened the project that had an issue and build it. That worked for me.

Make sure you close all instances of Visual Studio and then restart as Administrator. In my case I had multiple instances of Visual Studio open and I had to close them all and then restart Visual Studio as admin in order to get things working.

I had visual studio 2015 with an ASP.net MVC5 arcitectured with DDD (Domain Driven Design). The problem was the same since I just opened visual studio normally.
The problem was solved when I closed all instance visual studio and restarted as an adminstrator.

I also faced to this issue in VS 2015 , but finally I resolved it by following bellow steps
In VS go to : Tools -> Options - > Projects and Solutions -> Build and Run
Select "Always Build" in "On Run when project are out of date"

This issue happened to me with the Latest VS2019.
I've tried everything here, but unfortunately without any result.
Finally, I've restarted the IIS server, also manually Recycled the "Default Application Pool".
After debugging issue fixed.

For me solution was to run testapp.csproj instead of testapp.sln. This can be selected from drop down next to green play button.

Simply updated visual studio which then prompted PC restart. This is what fixed the problem for me.
Note: I tried some of the above / below but to no avail.

I got this problem solved by clearing MEF component cache.
Use this VS Extension to clear cache very easily.!!

I stop debugging and exit the projects from IIS Express then clean and build the project and my error is gone.

I get this running as non-Adminsitrator in Visual Studio 2019 16.2.5, with two .NET Core projects.
The solution I found is to set at least one of the projects as "Start" (in Solution's Property Page). If both are set to "Start without Debugging" it fails. This happens even if I want to start the entire project without any debugging (Ctrl+F5).
Bug. Still unaddressed by Microsoft and their bug page issue report it is closed as unreproducible.

#Flowerking's commend did it for me.
Right click on you project and select "Set as StartUp Project".

I had this problem developing a react native app for windows in Visual Studio 2019. I started the debugger then stopped it in the terminal by using ctrl-C. I was not able to restart the debugger and the "start debugging" and "start without debugging" options were disabled in the debug dropdown. Restarting Visual Studio, etc, and some of the things described above did not work. Through hunting and testing, I noticed that if I switched views in the Solution Explorer to the MyProject.sln instead of the fileview, the debugging was enabled again. I'm not sure why, but it appears I need to be in the sln view to start the debugger.

Visual Studio Code IntelliSense suggestions don't pop up automatically

I followed the install instructions in https://code.visualstudio.com, but when I write C# code, the IntelliSense suggestions don't pop up automatically, so I must trigger it by using shortcut Command+Space, meanwhile, the suggestions are just from the code I wrote.
Here is the screen capture below:
I checked in https://code.visualstudio.com/Docs/editingevolved, VSCode do support C# IntelliSense.
I don't think it is bug, maybe I did something wrong when installing VSCode. How to solve this problem?

To activate IntelliSense on a .NET project:
Open project folder: File->Open folder...
Click on the flame on your bottom left corner, pick your project
IntelliSense activated!
From what I've seen, IntelliSense cannot be activated on a .cs file alone.
(See #natemcmaster's answer).

OmniSharp doesn't know where your project files are. In your screenshot, the left sidebar says "You have not opened a folder". OmniSharp will look in your currently opened folder for project files.
Add your folder. File > Open Folder...
Make sure you have a project file (either project.json or your *.csproj files).
This should be enough for OmniSharp to start providing you Intellisense.

Try changing the editor.quickSuggestions setting to true in the settings.json file. The settings file may be found in:
Windows: %APPDATA%\Code\User\settings.json
Mac: $HOME/Library/Application Support/User/settings.json
Linux: $HOME/.config/Code/User/settings.json

You just have to pick up the language type from the bar at the bottom (the button with the flame).

In my case the problem was that I had 2 versions installed. I was launching from the command prompt and it kept running the older version. In Programs and Features, I removed the one that didn't have "(user)" at the end of the application name, and when I opened Code again, it started downloading packages, one of which was OmniSharp, and then IntelliSense started working.
Note that I was also getting the following warning when opening the older version, and searching for the warning led me to instructions on removing the other version, which in turn fixed the IntelliSense problem:
You are running the system-wide installation of Code, while having the user-wide distribution installed as well. Make sure you're running the Code version you expect.

Issue :
if i hit ! it was not showing any suggestion(emmet), i also tried to hit ctrl+space its was showing "no suggestions". i tried to uninstall and re- install vs code several times but seems its picking the previous settings somehow(for example old extensions are pre loaded with every fresh install).
So, i went to below path and deleted .vscode folder and it fixed the issue.
Steps:
uninstall vscode
go to below path
C:\Users<user_name_of_the_system>\
and delete .vscode folder
restart the system and install vs code
You will be all set..!!
Note : this is applicable only for windows pc

Visual Studio C# IntelliSense not automatically displaying

Just recently, my Visual Studio 2010 stopped displaying IntelliSense suggestions automatically while I am typing. I can still press ctrl+space to get it to work, but it doesn't automatically show a list of suggestions like it used to. I have already tried disabling all my extensions, restarting VS and the computer, and I have checked all the appropriate settings (Options -> Text Editor -> C# -> IntelliSense) to make sure that it is set to offer suggestions after a character is typed.
Has anyone seen this behavior before? Does anyone have any other suggestions for how I can get IntelliSense to go back to the way things were before? If not, I might just have to do a fresh reinstall of VS...

In prose, in case you can't see the above image:
Open Tools > Options > Text Editor. If you're only having this issue with one language, find that language; if it's for everything, click "All Languages". Right at the top, there'll be a few options labeled "Auto list members", "Hide advanced members", and "Parameter information". Make sure all of those are enabled (though the second may be disabled; if so, ignore it).

I have found that at times even verifying the settings under Options --> Statement Completion (the answer above) doesn't work. In this case, saving and restarting Visual Studio will re-enable Intellisense.
Finally, this link has a list of other ways to troubleshoot Intellisense, broken down by language (for more specific errors).
http://msdn.microsoft.com/en-us/library/vstudio/ecfczya1(v=vs.100).aspx

I'll start off my noting that this hasn't happened since I upgraded my RAM. I was at 4GB and would often have multiple instances of VS open along with SSMS. I have since gone to 8GB and then 16GB.
Here's the steps I go through when I lose intellisense.
If only one file/window appears to be affected, close/reopen that file. If that doesn't work, try below.
In Visual Studio:
Click Tools->Options->Text Editor->All Languages->General
Uncheck "Auto list members"
Uncheck "Parameter information"
Check "Auto list members" (yes, the one you just unchecked)
Check "Parameter information" (again, the one you just unchecked)
Click OK
If this doesn't work, here's a few more steps to try:
Close all VS documents and reopen
If still not working, close/reopen solution
If still not working, restart VS.
For C++ projects:
MSDN has a few things to try: MSDN suggestions
The corrupt .ncb file seems most likely.
From MSDN:
Close the solution.
Delete the .ncb file.
Reopen the solution. (This creates a new .ncb file.)
Notes:
This issue does not appear to be specific to C# as C++ and VB users
report the same issue
Tested in VS 2013/2015

Steps to fix are:
Tools
Import and Export Settings
Reset all settings
Back up your config
Select your environment settings and finish

I also faced the same issue but in VS2013.
I did the below way to fix, It was worked fine.
Close all the opened Visual studio instance.
Then, go to "Developer command prompt" from visual studio tools,
Type it as devenv.exe /resetuserdata
Restart the machine, Open the Visual studio then It will ask you to choose the development settings from initial onwards, thereafter open any solution/project. You'll be amazed.
Hope, it might helps you :)

Deleted the .suo file in solution folder to solve the problem.

Sometimes i've found Intellisense to be slow. Hit the . and wait for a minute and see if it appears after a delay. If so, then I believe there may be a cache that can be deleted to get it to rescan.

I hit this today after the following sequence:
Added a new class to my project.
Closed Visual Studio, but accidentally selected No when it asked if I wanted to save changes.
Reopened Visual Studio, and found that it reopened the new file automatically but without my previous changes (as expected). However, IntelliSense was no longer working in the new file.
The problem was in addition to not saving changes to the new file, it didn't save changes to the project, so after reopening Visual Studio the file was not part of the project. The Show All Files command in Solution Explorer, or Add → Existing Item..., resolved the problem.

I had the file excluded from the project so i was not able to debug and have intellisense on that file.
Including the file back into the project solved my problem! :)

A new cause for this in the .net core era is having a project loaded for an unsupported .net core version. For instance if you loaded a project from GitHub that was set to use:
<PropertyGroup>
<TargetFramework>netcoreapp2.2</TargetFramework>
<AspNetCoreHostingModel>InProcess</AspNetCoreHostingModel>
</PropertyGroup>
But you only have 2.1 installed or find yourself using Visual Studio 2017 then the compiler wont be able to find the SDK code and thus provide intellisense.
The solution in that case might be to right click on your project and select Edit MyProject.csproj from the context menu and change the target framework as necessary:
<PropertyGroup>
<TargetFramework>netcoreapp2.1</TargetFramework>
<AspNetCoreHostingModel>InProcess</AspNetCoreHostingModel>
</PropertyGroup>
This assumes whatever project you loaded can actually be run under a lesser target framework.

I simply closed all pages of visual studio and reopened ..it worked.

Deleting the .vs folder in the solution solved my issue. You have to exit from Visual Studio and then delete the .vs folder and start Visual Studio again.

Closed all my VS windows
Started the Visual Studio Installer and clicked 'Modify'.
Under 'Individual components' > 'Code Tools' > Deselected NuGet package manager and re-selected it.
After modifying and restarting VS, IntelliSense was working correctly again.
Found my answer on https://developercommunity.visualstudio.com/content/problem/130597/unity-intellisense-not-working-after-creating-new-1.html

[Tools -> Options -> Text Editor -> All Languages -> CodeLens]
Check if check box "Enable CodeLens" is checked

I have just come to about this problem while installing one of the extensions and its file was deleted by my anti virus so I just disabled my anti virus and reinstalled visual studio. Suggestions are working properly without any changes made after installation.

At the bottommost right look at the blue line where Ln, Col, Spaces, UTF, CRLF,..... here the language is specified.
Check that your language and the language specified there are the same.
In my case, it was Django Python while I was trying to use HTML.

This may be due to the solution configuration changed to Release Mode instead of debug. Right click on solution -> Properties -> Configuration Properties -> Set Configuration To Debug if it is in Release.