## Why is this an issue?

Composite format strings in C# are evaluated at runtime, which means they are not verified by the compiler. Introducing an ill-formed format item, or indexing mismatch can lead to unexpected behaviors or runtime errors. The purpose of this rule is to perform static validation on composite format strings used in various string formatting functions to ensure their correct usage. This rule validates the proper behavior of composite formats when invoking the following methods:

- `String.Format`
- `StringBuilder.AppendFormat`
- `Console.Write`
- `Console.WriteLine`
- `TextWriter.Write`
- `TextWriter.WriteLine`
- `Debug.WriteLine(String, Object[])`
- `Trace.TraceError(String, Object[])`
- `Trace.TraceInformation(String, Object[])`
- `Trace.TraceWarning(String, Object[])`
- `TraceSource.TraceInformation(String, Object[])`

### Noncompliant code example

    s = string.Format("[0}", arg0); // Noncompliant: square bracket '[' instead of curly bracket '{'
    s = string.Format("{{0}", arg0); // Noncompliant: double starting curly brackets '{{'
    s = string.Format("{0}}", arg0); // Noncompliant: double ending curly brackets '}}'
    s = string.Format("{-1}", arg0); // Noncompliant: invalid index for the format item, must be >= 0
    s = string.Format("{0} {1}", arg0); // Noncompliant: two format items in the string but only one argument provided

### Compliant solution

    s = string.Format("{0}", 42); // Compliant
    s = string.Format("{0,10}", 42); // Compliant
    s = string.Format("{0,-10}", 42); // Compliant
    s = string.Format("{0:0000}", 42); // Compliant
    s = string.Format("{2}-{0}-{1}", 1, 2, 3); // Compliant
    s = string.Format("no format"); // Compliant

### Exceptions

The rule does not perform any checks on the format specifier, if present (defined after the `:`). Moreover, no issues are raised in the following cases:

- the format string is not a `const`.

      var pattern = "{0} {1} {2}";
      var res = string.Format(pattern, 1, 2); // Compliant, non-constant string are not recognized

- the argument is not an inline creation array.

      var array = new int[] {};
      var res = string.Format("{0} {1}", array); // Compliant the rule does not check the size of the array

## Resources

### Documentation

- Composite formatting
- String.Format
- StringBuilder.AppendFormat
- Console.Write
- Console.WriteLine
- TextWriter.Write
- TextWriter.WriteLine
- Debug.WriteLine
- Trace.TraceError
- Trace.TraceInformation
- Trace.TraceWarning
- TraceSource.TraceInformation
- Standard format strings