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How to implement partial class in MVC4?

I have a class Employee.cs that was autogenerated by EntityFramework when i connected to a database, now i want to implement a partial class but I can't use "Employee.cs" name for that class because it already exists. Can I give other name to it and it will work? Can you explain how the compiler will know that there is another partial class of Employee.cs?

You can give the file whatever name you want like Employee.custom.cs but you have to call the class in it Employee in the same namespace as the Employee class in the other file, also with a partial modifier, and you should be good to go.
The compiler just gathers all the partial classes with the same name and compiles it into one class per name.

Compiler always works with fullname of class. If there is a project named MyProject, full name for the class would be something like:
MyProject.Employee
If You want to create class with the same name, You have to add a level of separation in naming, or if it have to be partial class with this generated class, You have to mark it also as a partial.
You can either create (in Model/Employee.cs):
public class MyProject.AnySubPath.Employee {}
Or:
In file Model/Employee.cs
public partial class MyProject.Employee {}
In file ViewModel/Employee.cs
public partial class MyProject.Employee {}
Important for You is to realize what is the partial mean & what will the compiler to with Your class: In fact the compiler will just put all the partial classes together and then compile them. Only benefit from being partial is that You can have the code split among multiple files. Excelent example for this is Windows.Forms, where You have "code-behind" file (Form1.cs) and then You have "designer" file (Form1.Designer.cs).
Also You can name the files whatever name You want, it is just common standard to have 1 class = 1 file and the class name should fit the file name.

How do I make these errors go away?

Can you tell me please what I need to change so that I won't get these errors anymore? I've duplicated an xaml+css class and that's why I get so many errors. I've attached an image to show you the errors. Someone recommended me to change something about these 2 things but I didn't quite understand, which is why I've decided to make a new post regarding this matter.
code c1.cs:
public partial class ImagesGrid : UserControl
code xaml c1.xaml:
x:Class="KinectInfoPortal.ImagesGrid"
EDIT1:
Code for C1.xaml.cs class:
public partial class ImagesGrid : UserControl
Code for ImageGrid.xaml.cs class:
public partial class ImagesGrid : UserControl
Code for C1.xaml:
x:Class="KinectInfoPortal.ImagesGrid"
Code for ImageGrid.xaml:
x:Class="Microsoft.Samples.Kinect.ControlsBasics.ImagesGrid"

Here is the mistake:
Code for C1.xaml.cs class:
public partial class ImagesGrid : UserControl
Code for ImageGrid.xaml.cs class:
public partial class ImagesGrid : UserControl
It should be:
Code for C1.xaml.cs class:
public partial class C1: UserControl
Code for ImageGrid.xaml.cs class:
public partial class ImagesGrid : UserControl
You renamed file to C1.xaml but your class name is same as it is for ImagesGird.xaml user control due to which it is saying that its already defined changed class name as well.
You have two class with same name.
1) in ImagesGrid.xaml user control
2) in C1.xaml user control
which is obviuosly wrong.

When you duplicate a xaml/cs pair, you need to take care to change the name of the class in both the XAML file and the codebehind, basically the two lines of code you listed. Make sure that in the original class and in the new class, BOTH are different.

Missing partial modifier on declaration of type [class name]; another partial declaration of this type exists

Note: I found some similar questions (Link 1 and Link 2), but they didn't helped me.
I'm trying to create a partial class, but I getting the following error on the "main class":
Missing partial modifier on declaration of type SomeClass; another partial declaration of this type exists
Those are the files that cotains the classes and the problem is that the error is pointing to the "main class" not the partial class. The reason for that I'm creating this partial class is that the file SomeClass.cs is Checked Out with someone else as we (the people developing with me) are using Team Foundation Server for source control.
File: SomeClass.cs
namespace MyNamespace
{
public class SomeClass
{
// Some methods...
}
}
File: SomeClass2.cs
namespace MyNamespace
{
public partial class SomeClass
{
// Some methods...
}
}

If you check MSDN - Partial Classes and Methods you can find the notice: All the parts must use the partial keyword.

Just flag both declarations as partial.
There is no "main" class and "partial" classes when you use partial in C#. The partial modifier just means that the class can be defined in multiple locations, but it is still a single class. The compiler wants all declarations of the class to be denoted as partial if any are partial, which has the effect of always making every location obvious that there is more to the class declaration than what you see at the time.

If any declaration of a class is labeled partial, then all declarations of it need to be partial.

Accessing partial class variables

Hi I have a partial class which reads a value from in
public partial class getFile : System.Web.UI.Page
{
string file = AcceptButton.FileName;
}
string file I want to access in a regular class
public class
{
string _file=file;
}
First class being partial class I cant do that. How can I do this?

First class being partial class I cant do that. How can I do this?
The fact that the class is partial has nothing to do with this. From the point of view of an external type it doesn't know (or care) that the class is a partial class.
The reason you can access file from some other class is that file is private.
To access the member directly you would need that member to be public, and you'd also need an instance of that type.
Having said that, since it seems this is related to as webpage, it doesn't make sense for some other class to be accessing the pages fields in the first place. What you should be doing is having the constructor or method(s) of test accept a parameter of type string to which getFile can pass in it's file field when either creating a test object or calling one of its methods.

partial class does not contain definition

I got a weird problem and finally fixed it. Just like to share this expeirence so other people will search this topic and get the answer.
I defined 2 partial classes, and compiled with the error of "does not contain a definition for ...". It was weird because it worked when the 2 classes were individual classes.
Here is the code:
Before:
Foo1.cs
public class foo1{public void xyz(){}}
Foo2.cs
public class foo2{}
After:
Foo1.cs
public partial class foo1{ public void xyz(){}}
Foo2.cs
public partial class foo1{}
Compilation said "MyNamespace.foo1 does not contain a definition for xyz". But xyz was good when the class was not partial! This could be caused by different namespaces. But I doubted that. Because if namespaces were different, it would not compile.
I figured out at last. The problem was in Foo1.cs, the namespace was not defined but it was defined in Foo2.cs. Before I defined the partial class, the compiler provided a default namespace to Foo1.cs, that was the same as the one defined explicitly in Foo2.cs. But with the partial class, the compiler would not provide a default namespace to the partial class. This is why the compiler complained when a third party called foo1.xyz().
I learned a lesson that always define the namespace explicitly for each single cs file, except that you do not define the namespace.

Always define namespace explicitly.