# Querying and filtering

Source: https://libtmux.org/en/csharp/latest/guides/querying-and-filtering/

> Choose a target and handle missing or ambiguous results.

[`Server.HasSessionAsync`](<https://libtmux.org/en/csharp/latest/reference/libtmux-server-hassessionasync/>) checks whether a session exists. It does not return
a selected session object. The complete program checks `work`, reports its
absence and leaves the borrowed server running. Use collection filtering when
you need handles or must detect several matching objects.

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## Require exactly one match

[Attaching to tmux](https://libtmux.org/en/csharp/latest/guides/attaching-to-tmux/) provides the complete C# program
and its setup. It searches an existing server for `work`, prints the name and
reports an absent session. Its launcher checks that the server remains running.

For a more general predicate, decide whether zero or several results are valid
before indexing a collection. Keep lookup and command failures visible: another
client can change the server between a read and the operation using its result.

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## Declarative filters

[Filtering and queries](https://libtmux.org/en/csharp/latest/concepts/queries/) describes this port's query APIs,
accepted fields and result-count contracts. Use that contract when storing a
query in configuration.

## Case-insensitive matching

Choose case handling explicitly when your query needs it. The attach program
uses exact case because the intended session is named `work`.

## Push the filter into tmux, or read once and filter locally

A tmux-side filter reduces returned rows; a captured collection can answer
several local queries from one read. Neither reserves the result. Check
unexpectedly empty format values before assuming that an object does not exist.