C# error decoder

Search brings you here with a code like error CS0029 and a deadline. Each page below makes the error happen live in your browser, explains the cause in plain English, and hands you the fix to copy.

Syntax errors

• CS1002 — ; expected
  The most common C# error. The real mistake is on the line before the one the error points at — plus what does and doesn't need a semicolon.
• CS1513 — } expected
  A block was never closed. The error line is where the compiler gave up, not where the brace belongs — brace-counting ritual included.
• CS1525 — Invalid expression term 'X'
  A value slot holding punctuation: an empty initializer, a dangling operator, empty parens. Five shapes plus the cascade check.
• CS1022 — Type or namespace definition, or end-of-file expected
  One closing brace too many — the mirror of CS1513. Brace counting finds it in seconds.
• CS1519 — Invalid token in class, struct, or interface member declaration
  A statement where a member should be, or a field with no type. Why executable code must live in a method.
• CS1001 — Identifier expected
  A name slot got a value, or a keyword was used as a name. The five name slots and the @ escape.
• CS1031 — Type expected
  new 5(), empty generics, parameters with no type — the type-shaped sibling of CS1001.

Type conversion errors

• CS0029 — Cannot implicitly convert type 'X' to 'Y'
  You put a string where a number belongs (or vice versa). Four causes, four fixes, plus the difference from CS0266.
• CS0019 — Operator cannot be applied to operands of type 'X' and 'Y'
  The operator has no version for those two types: subtracting from a string, comparing int with string, adding to a bool.
• CS0021 — Cannot apply indexing with [] to an expression of type 'X'
  Square brackets on something that holds one value, not many. What is actually indexable in C#, and the number→digit patterns.
• CS1955 — Non-invocable member 'X' cannot be used like a method
  Parentheses on a property: list.Count() should be list.Count. The action-vs-value rule that decides it.

Name resolution errors

• CS0103 — The name 'X' does not exist in the current context
  Typo, use-before-declaration, or the braces-scope trap. The three causes cover almost every occurrence.
• CS0246 — The type or namespace name 'X' could not be found
  Unknown type (not variable): misspelled class, missing using, or an unreferenced package. Decision table included.
• CS1061 — 'X' does not contain a definition for 'Y'
  The variable is fine; the part after the dot isn't. Typos, wrong casing, the wrong type's member, and calling a property like a method.
• CS0117 — 'X' does not contain a definition for 'Y' (static members)
  The type-level twin of CS1061: a typo after the class name, like Console.WritLine or Math.Sqr.

Flow-control errors

• CS0165 — Use of unassigned local variable 'x'
  Read before written — C# refuses to hand you a silent zero. Why assignment inside one if branch isn't enough, and the three fixes.
• CS0163 — Control cannot fall through from one case label to another
  C# refusing the C-era switch bug. Every case ends in break/return/throw — and how stacked labels give you the "fall-through" you actually wanted.
• CS0161 — Not all code paths return a value
  A non-void method with a branch that falls off the end. The if/else, trailing-return and switch-default fixes.
• CS1503 — Argument N: cannot convert from 'X' to 'Y'
  The method-argument sibling of CS0029. Convert at the call site, or fix the signature.
• CS0122 — 'X' is inaccessible due to its protection level
  You reached for a private member from outside. The property/method fixes — and why not to just make everything public.

Declarations & assignment

• CS0128 — A local variable named 'X' is already defined
  Declared the same name twice in one scope — usually a copy-paste leftover carrying its type keyword. Assign vs re-declare, and why different scopes may share names.
• CS0131 — The left-hand side of an assignment must be a variable
  Nowhere to store: a constant, a literal, or an expression sat left of '='. What the left side is allowed to be.

Method & constructor calls

• CS1501 / CS7036 — wrong number of method arguments
  Your call and the parameter list disagree on count. Optional parameters, overloads, and named arguments as the rescue.
• CS1729 — 'X' does not contain a constructor that takes N arguments
  Declaring any constructor removes the free empty one — why new X() suddenly breaks, and the two fixes.

Coverage

That's 24 of the most common beginner-facing compiler errors, each with a live repro. Ran into one that's not here? Tell us and it moves to the front of the queue.