A value is assigned to a local variable, but either that variable is never read later on, or its value is always overwritten before being read. This means that the original assignment has no effect, and could indicate a logic error or incomplete code.

Ensure that you check the program logic carefully. If a value is really not needed, consider omitting the assignment. Be careful, though: if the right-hand side has a side effect (like performing a method call), it is important to keep this to preserve the overall behavior.

The following example shows six different types of assignments to local variables whose value is not read:

In ParseInt, the result of the call to int.TryParse is assigned directly to the unread local variable success.
In IsDouble, the out argument of the call to int.TryParse is assigned to the unread local variable i.
In ParseDouble, the exception thrown by the call to double.Parse in case of parse failure is assigned to the unread local variable e.
In Count, the elements of ss are assigned to the unread local foreach variable s.
In IsInt, o is assigned (in case o is an integer) to the unread local type test variable i.
In IsString, o is assigned (in case o is a string) to the unread local type case variable s.

The revised example eliminates the unread assignments.

Wikipedia: Dead store.

MSDN, Code Analysis for Managed Code, CA1804: Remove unused locals.

Microsoft: What's new in C# 7 - Discards.