The Secret of Curly Braces in C#: How to Properly Escape in String.Format

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Why are curly braces in C# a headache?

Have you ever written code in C#, wanted to output text like {Name}: {Value}, but instead got a FormatException? If so — you are not alone. The question “How to escape curly brace in String.Format” on Stack Overflow has gathered tens of thousands of views. And this is no coincidence: curly braces {} in C# have a special meaning — they denote a placeholder for variable substitution.

But what if you need to output the braces themselves as regular characters? For example, for JSON, templates, code generation, or just for nicely formatting a report? Today we will break down this topic from start to finish: from simple escaping to advanced tricks with StringBuilder and string interpolation.

How to escape curly braces: the basic rule

The answer is simple and elegant: double the brace. If you want to output a single opening brace {, write {{. For a closing brace } — write }}. C# will understand that this is not a placeholder, but a regular character.

Here is the simplest example:

string result = string.Format("Value in curly braces: {{0}}", 42);Console.WriteLine(result);// Output: Value in curly braces: {0}

Note: {{0}} is not a placeholder, but an escaped pair of braces with the digit 0 inside. If we had written {0}, we would have gotten the variable value (the number 42).

Typical beginner mistake

Beginners often write like this:

// ERROR!string bad = string.Format("Hello, {name}!", name);// FormatException: Input string was not in a correct format.

Why? Because C# expects a numeric index inside the braces ({0}, {1}), not a variable name. For names, you need to use string interpolation (see below) or explicitly specify indices.

Practical examples: from simple to complex

Let's break down several real-world scenarios where escaping braces is necessary.

1. Outputting JSON or XML for debugging purposes

Imagine you are writing a logger and want to output a JSON string with a substituted value:

int userId = 101;string json = string.Format("{{\\"userId\\": {0}, \\"name\\": \\"Alice\\"}}", userId);Console.WriteLine(json);// Output: {"userId": 101, "name": "Alice"}

Here {{ and }} create the outer curly braces of the JSON, and {0} substitutes the number.

2. Generating code templates

Suppose you are writing a code generator in C#. You need to create a string with a template for another language (e.g., Python or JavaScript):

string template = string.Format("def {0}():    return {{0}}", "calculate");Console.WriteLine(template);// Output:// def calculate()://     return {0}

Note: {{0}} in the output string is no longer a C# placeholder, but part of the generated code (e.g., for Python).

3. Formatting with multiple placeholders and braces

When you need to mix substitutions and escaping, the key is not to get confused with the indices:

string name = "Bob";int age = 30;string message = string.Format("User {{ {0} }} (age: {1})", name, age);Console.WriteLine(message);// Output: User { Bob } (age: 30)

Here {{ gives {, }} gives }, and {0} and {1} substitute the values.