C# Interview questions and answers Data Type Variable

What is C#?

C# is a modern, object-oriented programming language developed by Microsoft as part of the .NET platform.

What are the different types of variables in C#?

• Value types: int, double, bool, char, enum, struct.
• Reference types: string, object, class, interface.
• Other types: var, dynamic.

Explain the difference between value types and reference types.

• Value types store the actual data, while reference types store references to the data.
• Value types are stored on the stack, while reference types are stored on the heap.
• Value types are passed by value, while reference types are passed by reference.

What is the difference between `int` and `int?` in C#?

• `int` is a non-nullable value type that cannot have a null value.
• `int?` (nullable int) allows the variable to have a null value in addition to integer values.

Explain the `var` keyword in C#.

• `var` is an implicitly typed variable whose type is inferred by the compiler based on the assigned value.
• It allows for cleaner code and reduces verbosity.

What is boxing and unboxing in C#?

• Boxing is the process of converting a value type to a reference type.
• Unboxing is the process of converting a boxed value back to its original value type.

What is the difference between `string` and `StringBuilder` in C#?

• `string` is an immutable type used to represent a sequence of characters.
• `StringBuilder` is a mutable type designed for efficient string manipulation.

What is a constant variable in C#?

• A constant variable is a variable whose value cannot be changed after initialization.
• It is declared using the `const` keyword.

What are implicit and explicit conversions in C#?

• Implicit conversions occur automatically and do not require explicit casting.
• Explicit conversions require explicit casting using the appropriate syntax.

What is the purpose of the `dynamic` keyword in C#?

• The `dynamic` keyword allows for late binding and dynamic typing in C#.
• It defers type checking until runtime, providing more flexibility at the cost of reduced type safety.

What is the difference between `readonly` and `const` in C#?

• `readonly` variables can be assigned a value either at the time of declaration or within a constructor, and their value cannot be changed afterward.
• `const` variables must be assigned a value at the time of declaration, and their value cannot be changed.

What are nullable value types in C#?

• Nullable value types, denoted by `type?`, allow value types to have a null value in addition to their normal range of values.
• They are useful when a value type needs to represent the absence of a value.

What is the difference between `is` and `as` operators in C#?

• The `is` operator checks if an object can be cast to a specified type, returning `true` or `false`.
• The `as` operator attempts to cast an object to a specified type, returning `null` if the cast fails instead of throwing an exception.

Explain the concept of null-coalescing operator (`??`) in C#.

• The null-coalescing operator provides a concise way to assign a default value when a nullable expression evaluates to `null`.
• It returns the left-hand operand if it is not `null`, otherwise it returns the right-hand operand.

What are the released data types in C#?

• `decimal`: Represents decimal numbers with higher precision for financial and monetary calculations.
• `DateTime`: Represents dates and times.
• `TimeSpan`: Represents a duration of time.
• `Guid`: Represents a globally unique identifier.
• `Nullable<T>`: Represents a value type that can also be assigned a null value.

Explain the concept of nullable value types in C#.

• Nullable value types allow value types to have a null value in addition to their normal range of values.
• They are useful when a value type needs to represent the absence of a value.
• The `Nullable<T>` structure is used to create nullable value types.

How do you declare and use an enum in C#?

• An enum is declared using the `enum` keyword, followed by the name and a set of named constants.
• Enums provide a way to define a set of related values, such as days of the week or status codes.
• Enum values can be accessed using dot notation, like `EnumName.EnumValue`.

What is the purpose of the `var` keyword in C#? How is it different from `dynamic`?

• The `var` keyword allows the compiler to infer the type of a variable based on the assigned value.
• It is resolved at compile-time and provides static typing.
• In contrast, `dynamic` allows for late binding and dynamic typing, with type checking deferred until runtime.

How do you convert between data types in C#?

• Implicit conversions occur automatically when the conversion does not involve a data loss.
• Explicit conversions can be done using casting or conversion methods like `Convert.ToInt32()` or `int.Parse()`.

How do you handle null values in C#?

• You can check for null values using the null-coalescing operator (`??`) or the null-conditional operator (`?.`).
• You can also use conditional statements like `if (variable != null)` or null-checking methods like `IsNullOrWhiteSpace()` for strings.

How do you format strings in C#?

• String formatting can be done using composite formatting with `string.Format()`, interpolation with `$"..."`, or using the `String.Format()` method.
• Additionally, you can use formatting placeholders like `{0}`, `{1}`, etc., to specify the position of arguments within the format string.

How do you work with date and time in C#?

• C# provides the `DateTime` struct for working with dates and times.
• You can perform various operations like adding or subtracting time, formatting dates, comparing dates, and extracting components using properties and methods of the `DateTime` struct.

Explain the difference between `StringBuilder` and `String` in C#.

• `StringBuilder` is used for efficient string manipulation when there are frequent modifications to a string.
• `StringBuilder` is mutable and provides methods like `Append()`, `Insert()`, and `Replace()` for modifying strings without creating new instances.
• `String` is immutable, meaning that once created, it cannot be modified.