Why does my Dapper multi-mapping leave the parent object null?

Question

Manish Shah · Answer

Dapper multi-mapping works by dividing each result row into column segments. It uses the splitOn value to decide where one mapped object ends and the next begins. If that boundary doesn’t match your SELECT column order, the objects can be built from the wrong columns—or a mapped object can come back null.

For a parent and child, put the parent columns first, followed by the child columns, and make the child’s key the split point:

const string sql = @"
    SELECT p.Id, p.Name, c.Id, c.Name
    FROM Parent p
    LEFT JOIN Child c ON c.ParentId = p.Id";

var parents = connection.Query<Parent, Child, Parent>(
    sql,
    (parent, child) =>
    {
        // Attach the mapped child; it can be null when the LEFT JOIN finds no match.
        parent.Child = child;
        return parent;
    },
    splitOn: "Id"
).ToList();

Here, splitOn: "Id" marks the second Id column—the start of the child segment. The first Id belongs to the parent. This relies on the explicit column order; using SELECT * can make the boundary ambiguous, especially when both tables have an Id column.

Also check that the generic types are in the same order as the column segments: Parent, Child for the query above. With a LEFT JOIN, a missing child may legitimately map to null, but the parent should still be present. If the parent itself is null, inspect the returned row and confirm its first parent column (usually the key) is non-null and that the query actually returns a row.

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Why does my Dapper multi-mapping leave the parent object null?

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