How to Resolve Error "CS0031: Constant value 'value' cannot be converted to a 'type'" in C#

The Compiler Error CS0031 is a Range Overflow error. The message states: "Constant value 'value' cannot be converted to a 'type'".

This error occurs when you attempt to assign a constant value (like a literal number or a const variable) to a variable type that is not capable of holding that specific value. Usually, this means the number is too large for the target type (e.g., trying to fit 300 into a byte) or has the wrong sign (e.g., trying to assign -1 to an uint).

Because the value is constant, the compiler detects this problem immediately during the build, preventing a potential runtime crash or data corruption.

Understanding Type Ranges

Every numeric type in C# has specific limits. If a constant value falls outside these limits, CS0031 is triggered.

Scenario 1: Value Too Large (Overflow)

The most common cause is assigning a number literal that exceeds the maximum value of the target variable.

Example of Mistake

Trying to assign 300 to a byte (which maxes out at 255).

public void ConfigureSettings()
{
    // ⛔️ Error CS0031: Constant value '300' cannot be converted to a 'byte'.
    // A byte only has 8 bits (0 to 255). 300 requires 9 bits.
    byte value = 300;
}

Solution: Use a Larger Type

Change the variable type to something that can hold the value, such as int or short.

public void ConfigureSettings()
{
    // ✅ Correct: 'int' can easily hold 300.
    int value = 300;
    
    // Alternatively, if you must use a byte, correct the value:
    byte smallValue = 255;
}

Scenario 2: Sign Mismatch (Negative to Unsigned)

C# supports Unsigned types (uint, ulong, ushort, byte). These types interpret all bits as positive magnitude. Assigning a negative literal to them is invalid.

Example of Mistake

public void ProcessIndex()
{
    // ⛔️ Error CS0031: Constant value '-1' cannot be converted to a 'uint'.
    // 'uint' stands for Unsigned Integer. It cannot store negative numbers.
    uint index = -1;
}

Solution

Either use a signed type (like int) or assign a valid positive number.

public void ProcessIndex()
{
    // ✅ Correct: Use 'int' if you need negative numbers
    int index = -1;
    
    // ✅ Correct: Or assign a valid positive literal
    uint positiveIndex = 1;
}

Scenario 3: Enum Underlying Types

By default, C# Enums are backed by int. However, you can change the backing type to byte or short. If you define an Enum member with a value that doesn't fit the backing type, CS0031 occurs.

Example of Mistake

// We declared this Enum to use 'byte' (Max 255)
public enum Status : byte
{
    Start = 1,
    Processing = 100,
    
    // ⛔️ Error CS0031: Constant value '999' cannot be converted to a 'byte'
    Error = 999
}

Solution

Change the backing type of the Enum to accommodate the larger values.

// ✅ Correct: 'short' (Max 32,767) can hold 999.
// Or simply remove ': short' to default to 'int'.
public enum Status : short
{
    Start = 1,
    Processing = 100,
    Error = 999
}

Conclusion

CS0031 is the compiler telling you that the math doesn't fit.

1. Check Limits: Verify the Min/Max values of the type you are assigning to.
2. Check Signs: Ensure you aren't putting negative numbers into uint, ulong, or byte.
3. Change Types: If the value is correct but the type is too small, upgrade the variable (e.g., byte -> int).