CS0029 — Cannot implicitly convert one type to another

Environment: C# static type checking.

Understand the conversion boundary

CS0029 occurs when the expression's type cannot be assigned implicitly to the destination. C# does not turn a string into an integer merely because its characters are digits. It also refuses some numeric conversions that could lose range or precision; the correct repair depends on the data contract.

Reproduce a real mismatch

// int quantity = "42"; // CS0029: string cannot be assigned to int

Validate text at the boundary rather than hiding the mismatch with a cast:

using System;
string input = "42";
if (!int.TryParse(input, out int quantity)) {
    Console.Error.WriteLine("Not a valid whole number");
    return;
}
Console.WriteLine(quantity + 1);

Expected output: 43 in a modern SDK-style project supporting top-level statements. Change input to "42x" and the program should take the validation branch instead of crashing.

Numeric conversions are a separate case

An assignment such as int small = 3000000000L; fails at compile time because the long value is outside int range. An explicit cast can discard information or, in a checked context, throw. Prefer preserving long if that represents the real domain. For nullable types, decide what null means before unwrapping; avoid using ! to silence a different warning without proof.

Repair checklist

1. Identify the expression type and destination type; do not guess from displayed text.
2. Decide whether you need parsing, a safe explicit conversion, a different destination type, or a mapping function.
3. Test valid, malformed, empty and out-of-range inputs.
4. Preserve validation errors as useful diagnostics; do not swallow them into a default zero.
5. Rebuild and add a regression test for the original failing input.

A cast such as (int)"42" is not a legal string-to-int conversion. Parsing is the correct mechanism for text.