Why Doesn't Casting an Integer to an Invalid Enum Value in C# Throw an Exception?

Casting Ints to Invalid Enum Values Without Exceptions

In C#, when casting an integer to an enum that does not match any defined values, the result is not an exception, but rather a value that is outside the defined range. This behavior is intentional and stems from the backing value type of enums.

Enum Value Storage

Enums are implicitly backed by a value type, typically an integer. This allows them to represent discrete values and to be used in operations such as comparisons and calculations. Since the underlying value type can store any integer, enums can handle values other than those explicitly defined as named members.

Casting Integers to Enums

When casting an integer to an enum, the resulting value is the integer's representation in the underlying value type. This means that even if the integer does not correspond to a named member of the enum, it is still cast to the enum type and assigned the integer value.

Example

Consider the following enum:

enum Beer
{
    Bud = 10,
    Stella = 20,
    Unknown
}

When casting the integer 50 to the Beer enum:

int i = 50;
var b = (Beer) i;

Console.WriteLine(b.ToString());

The result will be "50", indicating that the integer 50 was stored as the value of the enum. This output may seem unexpected, but it is a consequence of the underlying integer storage of enums.

Reasoning Behind This Behavior

The rationale behind this behavior is that it provides flexibility in operations involving enums. It allows for the representation of values that are not explicitly defined in the enum, which can be useful in scenarios such as storing flags or representing ranges of values.

Example

For instance, consider an enum representing days of the week:

enum DayOfWeek
{
    Monday = 1,
    Tuesday = 2,
    Wednesday = 3,
    Unknown
}

If a function needs to accept any day of the week as an argument, it can cast integers to the DayOfWeek enum and handle values even if they are not explicitly defined. This approach provides more flexibility than requiring the definition of specific named members for each possible value.

Additional Considerations

While this behavior can be beneficial in certain circumstances, it also warrants caution. Developers should be aware that casting integers to enums can result in unexpected values if not handled carefully. To ensure the validity of enum values, consider using validation methods or explicitly defining all expected values within the enum.

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