Skip to main content

How to Resolve Error "CS0145: A const field requires a value to be provided" in C#

The Compiler Error CS0145 is an initialization error. The message reads: "A const field requires a value to be provided".

In C#, a const (constant) is a value that must be known at compile-time. The compiler takes the value you provide and literally "bakes" it into the compiled code wherever that constant is used. Because this process happens during compilation, you cannot declare a constant without immediately giving it a value. You cannot wait to assign it in a constructor or a method later.

This guide explains the rules of constants and how to fix this error.

Understanding Compile-Time Constants​

When you declare a variable as const, you are making a specific promise to the compiler: "This name represents a value that will never change, and I can tell you what that value is right now."

This allows the compiler to perform optimizations, such as replacing the variable name MaxRetries with the literal number 3 directly in the binary code. If you do not provide the value 3 at the moment of declaration, the compiler cannot perform this substitution.

Scenario: Declaring Without Initializing​

This error commonly occurs when developers treat const fields like standard variables, expecting to assign them later in a constructor or startup method.

Example of error:

public class AppSettings
{
    // ⛔️ Error CS0145: A const field requires a value to be provided.
    // The compiler does not know what 'Version' is supposed to be.
    public const string Version;

    public AppSettings()
    {
        // Too late! Constants cannot be assigned at runtime.
        Version = "1.0"; 
    }
}

Solution 1: Assign a Value Immediately​

If the value is known and fixed (e.g., a math constant, a version string, or a configuration key), simply assign it on the same line as the declaration.

public class AppSettings
{
    // ✅ Correct: The value is provided immediately.
    public const string Version = "1.0";
}
note

The value must be a Constant Expression. It cannot be a function call or a non-const variable. It must be something the compiler can calculate (like "A" + "B" or 5 * 10).

Solution 2: Use readonly for Deferred Assignment​

If you do not know the value at compile-time (for example, if it depends on a database connection, user input, or a configuration file loaded at startup), you cannot use const.

Instead, use the readonly keyword. A readonly field acts like a constant in that it cannot be changed after initialization, but it allows you to assign the value one time inside the constructor.

public class AppSettings
{
    // ✅ Correct: 'readonly' allows declaration without immediate assignment
    public readonly string SessionId;

    public AppSettings()
    {
        // We calculate/generate the value at Runtime
        SessionId = System.Guid.NewGuid().ToString();
    }
}

Conclusion​

CS0145 enforces the definition of a "Constant."

  1. Check the Definition: Are you using const?
  2. Assign Immediately: If yes, you must use = value; on the same line.
  3. Switch to Readonly: If you need to calculate the value later (e.g., in a constructor), change const to readonly.

Table of Contents