What is the difference between explicitly and implicitly in C#?

In C#, the difference between "explicitly" and "implicitly" primarily relates to type conversions and how the compiler handles them. Implicit conversions are performed automatically by the compiler without requiring any special syntax, as they are considered safe and guarantee no data loss or exceptions. Explicit conversions, conversely, require a cast operator (()) to be specified by the developer because they might involve data loss or could potentially throw an exception at runtime.

Understanding Type Conversion in C

Type conversion, also known as type casting, is the process of changing a value from one data type to another. C# provides mechanisms for both built-in types and user-defined types to be converted. The choice between implicit and explicit conversion depends on the nature of the conversion and its safety guarantees.

Implicit Conversions

Implicit conversions are conversions that C# performs automatically because they are inherently safe and involve no risk of losing information or throwing an exception.

Characteristics:

• Automatic: The compiler handles the conversion without any explicit syntax from the programmer.
• Safe: Guaranteed not to lose data.
• No Exceptions: Predefined implicit conversions always succeed and never throw an exception. User-defined implicit conversions should behave in that way as well.
• Widening Conversions: Typically used for "widening" conversions, where a value is converted to a type that can hold a larger range of values (e.g., converting a smaller integer type to a larger one).

Examples:

• Numeric Widening:

  int myInt = 100;
  long myLong = myInt; // Implicit conversion from int to long
  // No data loss, long can hold any int value.

• Reference Type Conversions:

  class BaseClass { }
  class DerivedClass : BaseClass { }
  
  DerivedClass derived = new DerivedClass();
  BaseClass baseObj = derived; // Implicit conversion from DerivedClass to BaseClass
  // DerivedClass "is a" BaseClass.

Explicit Conversions

Explicit conversions require a cast operator (()) to be explicitly stated in the code. These conversions are necessary when there's a possibility of data loss or when the conversion could throw an exception. The developer takes responsibility for the potential risks.

Characteristics:

• Manual: Requires the (TargetType) cast operator.
• Potential Data Loss: Data might be truncated or altered (e.g., converting a long to an int if the long value exceeds the int range).
• Potential Exceptions: Can throw exceptions at runtime (e.g., InvalidCastException when casting an object to an incompatible type, or OverflowException with checked keyword for numeric conversions).
• Narrowing Conversions: Often used for "narrowing" conversions, where a value is converted to a type that might not be able to hold the entire range of the original type.
• User-Defined: If a custom conversion can throw an exception or lose information, define it as an explicit conversion.

Examples:

• Numeric Narrowing:

  long myLong = 2147483647L; // Max int value
  int myInt = (int)myLong; // Explicit conversion from long to int
  // No data loss in this specific case, but possible if myLong was larger.
  
  long anotherLong = 3000000000L; // Larger than max int
  int anotherInt = (int)anotherLong; // Data loss will occur (overflow)
  // The value of anotherInt will be -1294967296 due to overflow.

• Reference Type Conversions:

  object obj = "Hello World";
  string str = (string)obj; // Explicit conversion from object to string
  // This is safe if obj is indeed a string.
  
  object number = 123;
  string invalidStr = (string)number; // Throws InvalidCastException at runtime
  // number is an int, not a string.

Key Differences at a Glance

When to Use Which

• Use Implicit Conversions when the conversion is guaranteed to be safe and lossless. This makes your code cleaner and allows the compiler to handle straightforward type adaptations. For user-defined types, only define an implicit conversion if it truly behaves like a widening conversion that never fails or loses information.
• Use Explicit Conversions when there's a possibility of data loss, truncation, or a runtime exception. This forces the developer to acknowledge the potential risks by using the cast operator. For user-defined types, always define explicit conversions when the conversion could fail (e.g., due to invalid input) or result in information loss.

User-Defined Conversion Operators

C# allows developers to define custom implicit and explicit conversion operators for their own types. This enables seamless conversion between a custom type and another type (either a built-in type or another custom type).

Defining Conversion Operators:

Conversion operators are implemented as public static operator methods within the class.

public class Centimeter
{
    public double Value { get; set; }

    public Centimeter(double value)
    {
        Value = value;
    }

    // Implicit conversion from Centimeter to Meter (assuming Meter holds 100 CM)
    // This is safe because Meter can represent Centimeter values without loss.
    public static implicit operator Meter(Centimeter c)
    {
        return new Meter(c.Value / 100.0);
    }

    // Explicit conversion from Centimeter to Inch
    // This might be explicit due to potential precision loss or conceptual difference.
    public static explicit operator Inch(Centimeter c)
    {
        return new Inch(c.Value / 2.54);
    }
}

public class Meter
{
    public double Value { get; set; }
    public Meter(double value) { Value = value; }
}

public class Inch
{
    public double Value { get; set; }
    public Inch(double value) { Value = value; }
}

// Usage:
// Centimeter cm = new Centimeter(200);
// Meter m = cm; // Implicit conversion
// Inch inch = (Inch)cm; // Explicit conversion

Important Guideline for User-Defined Conversions:
When designing user-defined conversion operators, it is crucial to adhere to the established principles:

• User-defined implicit conversions should behave like predefined C# implicit conversions; they should always succeed and never throw an exception. This ensures predictable and safe automatic conversions.
• If a custom conversion can throw an exception or lose information, it must be defined as an explicit conversion. This forces the developer to consciously perform the conversion, acknowledging the potential risks.

For more in-depth information, you can refer to the official C# documentation on User-defined conversion operators.