Getting type information at runtime in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of getting type information at runtime in C#?

It helps you find out what kind of object you have while the program is running. This is useful when you don't know the type in advance and want to make decisions based on the object's type.

beginner

Which C# keyword or method returns the runtime type of an object?

The GetType() method returns the exact type of an object at runtime.

intermediate

How do you compare the runtime type of an object to a known type in C#?

Use obj.GetType() == typeof(SomeType) to check if the object's runtime type matches SomeType.

intermediate

What is the difference between GetType() and typeof() in C#?

GetType() is called on an object instance to get its runtime type. typeof() is used with a type name to get the Type object representing that type at compile time.

beginner

How can you get the name of an object's type at runtime?

Call obj.GetType().Name to get the simple name of the object's type as a string.

Which method do you use to get the runtime type of an object in C#?

A TypeOfObject()

B typeof()

C GetType()

D GetRuntimeType()

GetType() is called on an object instance to get its runtime type.

What does typeof(int) return in C#?

A The Type object representing the int type

B The runtime type of an int variable

C The string "int"

D An instance of int

typeof(int) returns the Type object representing the int type at compile time.

How do you check if an object is exactly of type string at runtime?

A obj is string

B obj.GetType() == typeof(string)

C obj.GetType().Name == "string"

D obj == string

Use obj.GetType() == typeof(string) to check exact type equality.

Which property gives you the simple name of a type in C#?

A Type.FullName

B Type.ToString()

C Type.SimpleName

D Type.Name

Type.Name returns the simple name of the type as a string.

If you want to get the full namespace and name of a type, which property do you use?

A Type.FullName

B Type.Namespace

C Type.Name

D Type.ToString()

Type.FullName returns the full namespace and type name.

Explain how to get and compare the runtime type of an object in C#.

Think about how you find out an object's type and then check if it matches a known type.

Describe the difference between GetType() and typeof() in C#.

One works on objects, the other works on type names.

Practice

1. What does the GetType() method do in C#?

easy

A. Changes the type of an object

B. Creates a new object of a specified type

C. Returns the exact runtime type of an object

D. Deletes the object from memory

Solution

1. Step 1: Understand the purpose of GetType()

   The GetType() method is used to find out the exact type of an object while the program is running.

2. Step 2: Differentiate from other actions

   It does not change, create, or delete objects; it only returns type information.

3. Final Answer:

   Returns the exact runtime type of an object -> Option C

4. Quick Check:

   GetType() returns type info [OK]

Hint: Remember: GetType() tells you what type an object is [OK]

Common Mistakes:

• Thinking GetType() changes the object
• Confusing GetType() with creating objects
• Assuming GetType() deletes objects

2. Which of the following is the correct syntax to get the type of an object obj in C#?

easy

A. obj.GetType()

B. GetType(obj)

C. typeof(obj)

D. obj.Type()

Solution

1. Step 1: Recall method call syntax

   In C#, to get the runtime type of an object, you call the method on the object: obj.GetType().

2. Step 2: Understand other options

   GetType(obj) is invalid syntax, typeof(obj) is incorrect because typeof expects a type name, not a variable, and obj.Type() does not exist.

3. Final Answer:

   obj.GetType() -> Option A

4. Quick Check:

   Call GetType() on the object [OK]

Hint: Use dot notation: object.GetType() [OK]

Common Mistakes:

• Using typeof with a variable instead of a type
• Trying to call GetType as a standalone function
• Using incorrect method names like Type()

3. What will be the output of the following code?

object val = 123;
Console.WriteLine(val.GetType().Name);

medium

A. Int32

B. int

C. Object

D. String

Solution

1. Step 1: Identify the runtime type of val

   The variable val is declared as object but holds the integer value 123, which is of type Int32 in C#.

2. Step 2: Understand GetType().Name output

   Calling val.GetType().Name returns the name of the actual runtime type, which is Int32.

3. Final Answer:

   Int32 -> Option A

4. Quick Check:

   Runtime type name = Int32 [OK]

Hint: GetType().Name shows exact type name [OK]

Common Mistakes:

• Expecting 'int' instead of 'Int32'
• Thinking it returns 'Object' because of variable type
• Confusing with string output

4. Identify the error in this code snippet:

object data = "hello";
if (data.GetType() == string)
{
    Console.WriteLine("It's a string!");
}

medium

A. Missing semicolon after Console.WriteLine

B. Comparing GetType() to a type name instead of a Type object

C. Using == instead of .Equals() for type comparison

D. GetType() cannot be used on object variables

Solution

1. Step 1: Analyze the type comparison

   The code compares data.GetType() to string, but string is a keyword, not a Type object.

2. Step 2: Correct way to compare types

   To compare types, use typeof(string) which returns the Type object for string. So the condition should be data.GetType() == typeof(string).

3. Final Answer:

   Comparing GetType() to a type name instead of a Type object -> Option B

4. Quick Check:

   Use typeof(TypeName) for type comparison [OK]

Hint: Compare with typeof(TypeName), not just type name [OK]

Common Mistakes:

• Using type name without typeof()
• Thinking == can't compare types
• Assuming GetType() is invalid on object

5. You want to write a method that accepts an object and prints a special message if the object is an integer or a string. Which code correctly uses runtime type information to do this?

void CheckType(object obj)
{
    if (obj.GetType() == typeof(int))
        Console.WriteLine("Integer detected");
    else if (obj.GetType() == typeof(string))
        Console.WriteLine("String detected");
    else
        Console.WriteLine("Other type");
}

hard

A. The code should compare obj.GetType().Name to string literals

B. This code should use obj is int instead of GetType()

C. GetType() cannot be used inside methods

D. This code correctly checks types using GetType() and typeof()

Solution

1. Step 1: Understand the method logic

   The method uses obj.GetType() to get the runtime type and compares it with typeof(int) and typeof(string) to identify the type.

2. Step 2: Verify correctness of type checks

   This is a valid and common way to check exact types at runtime in C#. Using GetType() with typeof() is correct and safe.

3. Final Answer:

   This code correctly checks types using GetType() and typeof() -> Option D

4. Quick Check:

   GetType() + typeof() = correct type check [OK]

Hint: Use GetType() == typeof(Type) for exact type checks [OK]

Common Mistakes:

• Thinking GetType() can't be used in methods
• Using 'is' when exact type match is needed
• Comparing type names as strings instead of Type objects