From e1c327c9e17fb84b68d27a435997d2baafa0c412 Mon Sep 17 00:00:00 2001
From: "Dailey, Alaric" <alaricd@pengdows.com>
Date: Tue, 20 Oct 2026 06:45:33 -0500
Subject: [PATCH] Prepare requires an open connection

IDbCommand.Prepare documents InvalidOperationException when "The
Connection is not set. -or- The Connection is not Open()." Prepare was
an empty method, so a command with no usable connection passed it and
failed only when it executed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
---
 DuckDB.NET.Data/DuckDBCommand.cs      |  6 +++++-
 DuckDB.NET.Test/DuckDBCommandTests.cs | 16 +++++++++++++++-
 2 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/DuckDB.NET.Data/DuckDBCommand.cs b/DuckDB.NET.Data/DuckDBCommand.cs
index 6d602614..bed08427 100644
--- a/DuckDB.NET.Data/DuckDBCommand.cs
+++ b/DuckDB.NET.Data/DuckDBCommand.cs
@@ -164,7 +164,11 @@ public async IAsyncEnumerable<RecordBatch> ExecuteArrowBatchesAsync([EnumeratorC
         }
     }
 
-    public override void Prepare() { }
+    /// <summary>
+    /// Statements are prepared when the command executes, so this only checks what IDbCommand.Prepare documents:
+    /// InvalidOperationException when "The Connection is not set. -or- The Connection is not Open()."
+    /// </summary>
+    public override void Prepare() => EnsureConnectionOpen();
 
     protected override DbParameter CreateDbParameter() => new DuckDBParameter();
 
diff --git a/DuckDB.NET.Test/DuckDBCommandTests.cs b/DuckDB.NET.Test/DuckDBCommandTests.cs
index b807f2b6..f4ec9a0a 100644
--- a/DuckDB.NET.Test/DuckDBCommandTests.cs
+++ b/DuckDB.NET.Test/DuckDBCommandTests.cs
@@ -17,4 +17,18 @@ public void SetCommandTextAndConnection()
         cmd.CommandText.Should().Be("Select 1");
         cmd.Connection.Should().Be(Connection);
     }
-}
\ No newline at end of file
+
+    // https://learn.microsoft.com/dotnet/api/system.data.idbcommand.prepare, Exceptions: InvalidOperationException,
+    // "The Connection is not set. -or- The Connection is not Open()."
+    [Fact]
+    public void PrepareThrowsWithoutAnOpenConnection()
+    {
+        new DuckDBCommand("SELECT 1").Invoking(c => c.Prepare()).Should().Throw<InvalidOperationException>();
+
+        using var closed = new DuckDBConnection(DuckDBConnectionStringBuilder.InMemoryConnectionString);
+        new DuckDBCommand("SELECT 1", closed).Invoking(c => c.Prepare()).Should().Throw<InvalidOperationException>();
+
+        new DuckDBCommand("SELECT 1", Connection).Invoking(c => c.Prepare()).Should().NotThrow();
+    }
+}
+