Hi there!

My first attempt to write a post in english. Please, forgive me in advance by my poor writing skills in english (worst than the spanish ones, if it is possible).


Regardless this will be a very short post, I don’t want to let pass the opportunity to remember to my future me, that when using Dapper, if the query should filter with the SQL clause IN using collection parameter, I shouldn’t write the parentheses in the SQL statement itself.

Imagine that you have a set of data that should be filtered to include only those records that are equal to a list of values for a given property. Something like…

SELECT * FROM [table] WHERE [Property1] IN ('A', 'B', 'C')

Very common, isn’t it?

Also, the list of values that I want to filter by is dynamic, and it is coming from a IEnumerable<string> collection in C#.

At last, imagine that you are using dapper to perform your queries to the database.


Approach 1 - Dynamic query construction using string manipulation classes

Weird. You don’t need to do this.

Dapper helps you to convert a collection into it’s representation using the parameter notation on the query itself, passing to the Query<> family methods an anonymous object with a named property matching the parameter indicated in the query statement.

Approach 2 - Use collection parameter on Dapper

So, you want to use the parameter system from Dapper. Great! Good choice.

In this approach, your code probably looks like:

// 'connection' has been initialized previously
var values = new string[] { "A", "B", "C" };
var entity = await connection.QueryAsync<AmazingEntity>(queryStatement, new { values });

… and you probably define your query as follows:

SELECT * FROM [table] WHERE [Property1] IN (@values)

Wait! Wrong statement. This one will throw an exception when will be executed.


Dapper automatically identify a string collection array passed by parameter and, instead of converting it to 'A', 'B', 'C' is converting it to ('A', 'B', 'C'), resulting in a statement like:

SELECT * FROM [table] WHERE [Property1] IN (('A', 'B', 'C'))

Note the double parentheses in the sentence. This query is invalid in SQL. And will throw an exception:

Incorrect syntax near ‘,’

So, to fix this, just remove the parentheses from your query and let Dapper do the rest for you.

SELECT * FROM [table] WHERE [Property1] IN @values