CA1065: Do not raise exceptions in unexpected locations

Cause

A method that is not expected to throw exceptions throws an exception.

Rule Description

Methods that are not expected to throw exceptions can be categorized as follows:

• Property Get Methods

• Event Accessor Methods

• Equals Methods

• GetHashCode Methods

• ToString Methods

• Static Constructors

• Finalizers

• Dispose Methods

• Equality Operators

• Implicit Cast Operators

  The following sections discuss these method types.

Property Get Methods

Properties are basically smart fields. Therefore, they should behave like a field as much as possible. Fields do not throw exceptions and neither should properties. If you have a property that throws an exception, consider making it a method.

The following exceptions are allowed to be thrown from a property get method:

• System.InvalidOperationException and all derivatives (including System.ObjectDisposedException)

• System.NotSupportedException and all derivatives

• System.ArgumentException (only from indexed get)

• KeyNotFoundException (only from indexed get)

Event Accessor Methods

Event accessors should be simple operations that do not throw exceptions. An event should not throw an exception when you try to add or remove an event handler.

The following exceptions are allowed to be thrown from an event accesor:

• System.InvalidOperationException and all derivatives (including System.ObjectDisposedException)

• System.NotSupportedException and all derivatives

• ArgumentException and derivatives

Equals Methods

The following Equals methods should not throw exceptions:

• System.Object.Equals

• M:IEquatable.Equals

  An Equals method should return true or false instead of throwing an exception. For example, if Equals is passed two mismatched types it should just return false instead of throwing an ArgumentException.

GetHashCode Methods

The following GetHashCode methods should usually not throw exceptions:

• GetHashCode

• M:IEqualityComparer.GetHashCode(T)

  GetHashCode should always return a value. Otherwise, you can lose items in the hash table.

  The versions of GetHashCode that take an argument can throw an ArgumentException. However, Object.GetHashCode should never throw an exception.

ToString Methods

The debugger uses System.Object.ToString to help display information about objects in string format. Therefore, ToString should not change the state of an object and it should not throw exceptions.

Static Constructors

Throwing exceptions from a static constructor causes the type to be unusable in the current application domain. You should have a very good reason (such as a security issue) for throwing an exception from a static constructor.

Finalizers

Throwing an exception from a finalizer causes the CLR to fail fast, which tears down the process. Therefore, throwing exceptions in a finalizer should always be avoided.

Dispose Methods

A System.IDisposable.Dispose method should not throw an exception. Dispose is often called as part of the clean up logic in a finally clause. Therefore, explicitly throwing an exception from Dispose forces the user to add exception handling inside the finally clause.

The Dispose(false) code path should never throw exceptions, because this is almost always called from a finalizer.

Equality Operators (==, !=)

Like Equals methods, equality operators should return either true or false and should not throw exceptions.

Implicit Cast Operators

Because the user is often unaware that an implicit cast operator has been called, an exception thrown by the implicit cast operator is completely unexpected. Therefore, no exceptions should be thrown from implicit cast operators.

How to Fix Violations

For property getters, either change the logic so that it no longer has to throw an exception, or change the property into a method.

For all other method types listed previously, change the logic so that it no longer must throw an exception.

When to Suppress Warnings

It is safe to suppress a warning from this rule if the violation was caused by an exception declaration instead of a thrown exception.

Related Rules

CA2219: Do not raise exceptions in exception clauses

See Also

Design Warnings