(Note: Most are collected from Internet. 绝大部分内容来自互联网)

1. What's the difference between Hashtable and Dictionary?

Hashtable and Dictionary  are collection of data structures to hold data as key-value pairs. The differences are:

1).Dictionary is generic type, hash table is not a  generic type. The Hashtable is a weakly typed data structure, so you can add keys and values of any Object Type to the Hashtable. The Dictionary class is a strongly types < T Key, T Value >  and you must specify the data types for both the key and value.

2).While perform operations Dictionary is faster because there is no boxing/unboxing (valuetypes don't need boxing)  while in Hashtable boxing/unboxing (valuetypes need boxing) will happened and which may have memory consumption as well as performance penalties.

3).An important difference is that Hashtable supports multiple reader threads with a single writer thread, while Dictionary offers no thread safety. If you need thread safety with a generic dictionary, you must implement your own synchronization or (in .NET 4.0) use ConcurrentDictionary<TKey, TValue>.

2. What's boxing and unboxing in C#?

Boxing:

Implicit conversion of a value type (int, char etc.) to a reference type (object), is known as Boxing. In boxing process, a value type is being allocated on the heap rather than the stack.

Unboxing:

Explicit conversion of same reference type (which is being created by boxing process); back to a value type is known as unboxing. In unboxing process, boxed value type is unboxed from the heap and assigned to a value type which is being allocated on the stack.

More: http://www.dotnet-tricks.com/Tutorial/csharp/HL1W121013-Understanding-Boxing-and-Unboxing-in-C

For Example

.// int (value type) is created on the Stack

.int stackVar = ;

.

.// Boxing = int is created on the Heap (reference type)

.object boxedVar = stackVar;

.

.// Unboxing = boxed int is unboxed from the heap and assigned to an int stack variable

.int unBoxed = (int)boxedVar;

Real Life Example

.int i = ;

.ArrayList arrlst = new ArrayList();

.

.//ArrayList contains object type value

.//So, int i is being created on heap

.arrlst.Add(i); // Boxing occurs automatically

.

.int j = (int)arrlst[]; // Unboxing occurs

 3.What is the GAC, and where is it located?

The GAC is the Global Assembly Cache. Shared assemblies reside in the GAC; this allows applications to share assemblies instead of having the assembly distributed with each application. Versioning allows multiple assembly versions to exist in the GAC—applications can specify version numbers in the config file. The gacutil command line tool is used to manage the GAC.

It is located at %winroot%\assembly

4.What is DLL Hell, and how does .NET solve it?

DLL Hell describes the difficulty in managing DLLs on a system; this includes multiple copies of a DLL, different versions, and so forth. When a DLL (or assembly) is loaded in .NET, it is loaded by name, version, and certificate. The assembly contains all of this information via its metadata. The GAC provides the solution, as you can have multiple versions of a DLL side-by-side.

5.Why are strings in C# immutable?

Immutable means string values cannot be changed once they have been created. Any modification to a string value results in a completely new string instance, thus an inefficient use of memory and extraneous garbage collection. The mutable System.Text.StringBuilder class should be used when string values will change.

6.What is the difference between a struct and a class?

Structs cannot be inherited. Structs are passed by value and not by reference. Structs are stored on the stack not the heap. The result is better performance with Structs.

7.What is a singleton?

A singleton is a design pattern used when only one instance of an object is created and shared; that is, it only allows one instance of itself to be created. Any attempt to create another instance simply returns a reference to the first one. Singleton classes are created by defining all class constructors as private. In addition, a private static member is created as the same type of the class, along with a public static member that returns an instance of the class.

8 How does one compare strings in C#?
In the past, you had to call .ToString() on the strings when using the == or != operators to compare the strings’ values. That will still work, but the C# compiler now automatically compares the values instead of the references when the == or != operators are used on string types. If you actually do want to compare references, it can be done as follows: if ((object) str1 == (object) str2) { } Here’s an example showing how string compares work:

9.How do I simulate optional parameters to COM calls?

You must use the Missing class and pass Missing.Value (in System.Reflection) for any values that have optional parameters

What do you know about .NET assemblies?

Assemblies are the smallest units of versioning and deployment in the .NET application. Assemblies are also the building blocks for programs such as Web services, Windows services, serviced components, and .NET remoting applications.

What’s the difference between private and shared assembly?

Private assembly is used inside an application only and does not have to be identified by a strong name. Shared assembly can be used by multiple applications and has to have a strong name.

What’s a strong name?

A strong name includes the name of the assembly, version number, culture identity, and a public key token.

How can you tell the application to look for assemblies at the locations other than its own install?

Use the directive in the XML .config file for a given application. < probing privatePath=c:\mylibs; bin\debug /> should do the trick. Or you can add additional search paths in the Properties box of the deployed application.

How can you debug failed assembly binds?

Use the Assembly Binding Log Viewer (fuslogvw.exe) to find out the paths searched.

Where are shared assemblies stored?

Global assembly cache.

C# provides a default constructor for me. I write a constructor that takes a string as a parameter, but want to keep the no parameter one. How many constructors should I write?
Two. Once you write at least one constructor, C# cancels the freebie constructor, and now you have to write one yourself, even if there is no implementation in

What is the difference between "Dispose()" and "Finalize()"

http://www.c-sharpcorner.com/UploadFile/nityaprakash/back-to-basics-dispose-vs-finalize/

Dispose
Garbage collector (GC) plays the main and important role in .NET for memory management so programmer can focus on the application functionality. Garbage collector is responsible for releasing the memory (objects) that is not being used by the application. But GC has limitation that, it can reclaim or release only memory which is used by managed resources. There are a couple of resources which GC is not able to release as it doesn't have information that, how to claim memory from those resources like File handlers, window handlers, network sockets, database connections etc. If your application these resources than it's programs responsibility to release unmanaged resources. For example, if we open a file in our program and not closed it after processing than that file will not be available for other operation or it is being used by other application than they can not open or modify that file. For this purpose FileStream class provides Dispose method. We must call this method after file processing finished. Otherwise it will through exception Access Denied or file is being used by other program.

Finalize
Finalize method also called destructor to the class. Finalize method can not be called explicitly in the code. Only Garbage collector can call the the Finalize when object become inaccessible. Finalize method cannot be implemented directly it can only be implement via declaring destructor. Following class illustrate, how to declare destructor. It is recommend that implement Finalize and Dispose method together if you need to implement Finalize method. After compilation destructor becomes Finalize method.

Now question is, When to implement Finalize? There may be any unmanaged resource for example file stream declared at class level. We may not be knowing what stage or which step should be appropriate to close the file. This object is being use at many places in the application. So in this scenario Finalize can be appropriate location where unmanaged resource can be released. It means, clean the memory acquired by the unmanaged resource as soon as object is inaccessible to application.

Finalize is bit expensive to use. It doesn't clean the memory immediately. When application runs, Garbage collector maintains a separate queue/array when it adds all object which has finalized implemented. Other term GC knows which object has Finalize implemented. When the object is ready to claim memory, Garbage Collector call finalize method for that object and remove from the collection. In this process it just clean the memory that used by unmanaged resource. Memory used by managed resource still in heap as inaccessible reference. That memory release, whenever Garbage Collector run next time. Due to finalize method GC will not clear entire memory associated with object in fist attempt.
Now question is, When to implement Finalize? There may be any unmanaged resource for example file stream declared at class level. We may not be knowing what stage or which step should be appropriate to close the file. This object is being use at many places in the application. So in this scenario Finalize can be appropriate location where unmanaged resource can be released. It means, clean the memory acquired by the unmanaged resource as soon as object is inaccessible to application.

Finalize is bit expensive to use. It doesn't clean the memory immediately. When application runs, Garbage collector maintains a separate queue/array when it adds all object which has finalized implemented. Other term GC knows which object has Finalize implemented. When the object is ready to claim memory, Garbage Collector call finalize method for that object and remove from the collection. In this process it just clean the memory that used by unmanaged resource. Memory used by managed resource still in heap as inaccessible reference. That memory release, whenever Garbage Collector run next time. Due to finalize method GC will not clear entire memory associated with object in fist attempt.