Render LIST, ARRAY, STRUCT, and MAP columns as JSON strings instead of #VALUE!

UnitTests/UnitTestDuckDbQueries.cs

@@ -1,5 +1,6 @@
 using ExcelDna.Integration;
 using System.Diagnostics.CodeAnalysis;
+using System.Text.Json;
 using xlDuckDb;
 using Xunit;
 
@@ -255,4 +256,51 @@ public void TestTimeStampTz()
         Assert.IsType<DateTime>(data[1, 0]);
         Assert.Equal(new DateTime(1992, 9, 20, 10, 30, 0), data[1, 0]);
     }
+
+    [Fact]
+    public void TestList()
+    {
+        var data = DuckDbHelper.ExecuteQuery("SELECT ['apple', 'banana', 'cherry']");
+        Assert.NotNull(data);
+        var json = Assert.IsType<string>(data[1, 0]);
+        var list = JsonSerializer.Deserialize<List<string>>(json);
+        Assert.NotNull(list);
+        Assert.Equal(3, list.Count);
+        Assert.Equal("apple", list[0]);
+        Assert.Equal("banana", list[1]);
+        Assert.Equal("cherry", list[2]);
+    }
+
+    [Fact]
+    public void TestNullList()
+    {
+        var data = DuckDbHelper.ExecuteQuery("SELECT NULL::INTEGER[]");
+        Assert.NotNull(data);
+        Assert.IsType<ExcelError>(data[1, 0]);
+        Assert.Equal(ExcelError.ExcelErrorNA, data[1, 0]);
+    }
+
+    [Fact]
+    public void TestStruct()
+    {
+        var data = DuckDbHelper.ExecuteQuery("SELECT {'id': 101, 'status': 'active'}");
+        Assert.NotNull(data);
+        var json = Assert.IsType<string>(data[1, 0]);
+        var dict = JsonSerializer.Deserialize<Dictionary<string, JsonElement>>(json);
+        Assert.NotNull(dict);
+        Assert.Equal(101, dict["id"].GetInt32());
+        Assert.Equal("active", dict["status"].GetString());
+    }
+
+    [Fact]
+    public void TestMap()
+    {
+        var data = DuckDbHelper.ExecuteQuery("SELECT map(['color', 'shape'], ['red', 'circle'])");
+        Assert.NotNull(data);
+        var json = Assert.IsType<string>(data[1, 0]);
+        var dict = JsonSerializer.Deserialize<Dictionary<string, string>>(json);
+        Assert.NotNull(dict);
+        Assert.Equal("red", dict["color"]);
+        Assert.Equal("circle", dict["shape"]);
+    }
 }

xlDuckDb/DuckDbHelper.cs

@@ -1,6 +1,8 @@
-using System.Diagnostics.CodeAnalysis;
+using System.Collections;
+using System.Diagnostics.CodeAnalysis;
 using System.Numerics;
 using System.Text;
+using System.Text.Json;
 using DuckDB.NET.Data;
 using DuckDB.NET.Native;
 using ExcelDna.Integration;
@@ -62,6 +64,7 @@ public static class DuckDbHelper
         var timeTzField = new bool[reader.FieldCount];
         var dateOnlyField = new bool[reader.FieldCount];
         var uuidField = new bool[reader.FieldCount];
+        var jsonSerializeField = new bool[reader.FieldCount];
 
         for (var i = 0; i < reader.FieldCount; i++)
         {
@@ -76,6 +79,8 @@ public static class DuckDbHelper
             timeTzField[i] = fieldType == typeof(DateTimeOffset);
             dateOnlyField[i] = fieldType == typeof(DateOnly);
             uuidField[i] = fieldType == typeof(Guid);
+            jsonSerializeField[i] = typeof(IList).IsAssignableFrom(fieldType) ||
+                                    typeof(IDictionary).IsAssignableFrom(fieldType);
         }
 
         // Add the first row of column names
@@ -127,6 +132,12 @@ public static class DuckDbHelper
                 {
                     rowData[i] = (reader.GetGuid(i)).ToString();
                 }
+                else if (jsonSerializeField[i])
+                {
+                    rowData[i] = reader.IsDBNull(i)
+                        ? (object)ExcelError.ExcelErrorNA
+                        : JsonSerializer.Serialize(reader.GetValue(i));
+                }
                 else
                 {
                     rowData[i] = reader.GetValue(i);