stride3d/stride · error · InvalidOperationException

Can only merge TriangleList.

Error message

Can only merge TriangleList.

What it means

MergeDrawData only supports merging meshes whose PrimitiveType is TriangleList. This error is thrown when a mesh in the merge group uses any other primitive type (lines, strips, fan, triangle lists with adjacency, etc.), because index/vertex concatenation logic assumes triangle-list topology.

Solutions

1. Convert the mesh to TriangleList (expand strips via degenerate-triangle insertion or index rewriting) before merging
2. Exclude non-TriangleList meshes from the merge group
3. Set meshDrawData.PrimitiveType = PrimitiveType.TriangleList when the underlying data is actually triangle-list

Example fix

// before
meshDrawData.PrimitiveType = PrimitiveType.TriangleStrip;
MergeDrawData(list);
// after
meshDrawData.PrimitiveType = PrimitiveType.TriangleList;
ConvertStripIndicesToList(meshDrawData); // expand strip indices
MergeDrawData(list);

bool allTriangleLists = meshes.All(m => m.PrimitiveType == PrimitiveType.TriangleList);
if (!allTriangleLists) throw new InvalidOperationException("Convert strips first.");

static bool IsTriangleList(MeshDrawData m) => m.PrimitiveType == PrimitiveType.TriangleList;

try { MergeDrawData(meshes); }
catch (InvalidOperationException ex) when (ex.Message.Contains("Can only merge TriangleList")) {
    // convert offending meshes or exclude them, then retry
}

When it happens

Trigger: Calling MergeDrawData (via GroupDrawData) with a MeshDrawData whose PrimitiveType is TriangleStrip, LineList, LineStrip, PointList, or TriangleListWithAdjacency instead of TriangleList.

Common situations: Merging debug line meshes or strip-based geometry with regular triangle meshes; procedurally generated geometry using strips; meshes imported with preserved primitive topology.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

• Non-matching vertex buffer declarations.
• The mesh Data needs to have index buffer
• Sorting not implemented for multiple vertex buffers by…
• meshData is not simple.
• msaaResolverShaderName

AI-assisted analysis of stride3d/stride@96fad776d2 (2026-09-14). Data as JSON: /api/errors/68c782a68a4756e1. Report an issue: GitHub.

Appendix: source

Thrown at sources/engine/Stride.Rendering/Extensions/MergeExtensions.cs:59

            for (int i = 0; i < meshDrawDatas.Count; i++)
            {
                var meshDrawData = meshDrawDatas[i];

                // This should not happen anymore
                if (i != 0)
                {
                    if (!meshDrawData.IsSimple())
                        throw new InvalidOperationException("Can only merge simple MeshDrawData.");

                    if (meshDrawData.VertexBuffers.Length != firstMeshDrawData.VertexBuffers.Length)
                        throw new InvalidOperationException("Non-matching vertex buffer declarations.");

                    if (!meshDrawData.VertexBuffers[0].Declaration.Equals(firstMeshDrawData.VertexBuffers[0].Declaration))
                        throw new InvalidOperationException("Non-matching vertex buffer declarations.");
                }

                if (meshDrawData.PrimitiveType != PrimitiveType.TriangleList)
                    throw new InvalidOperationException("Can only merge TriangleList.");

                // Update vertex/index counts
                totalVertexCount += meshDrawData.VertexBuffers[0].Count;
                if (hasIndexBuffer)
                {
                    if (meshDrawData.IndexBuffer != null)
                        totalIndexCount += meshDrawData.IndexBuffer.Count;
                    else
                        totalIndexCount += meshDrawData.VertexBuffers[0].Count;
                }
            }

            // Allocate vertex buffer
            var result = new MeshDraw { PrimitiveType = PrimitiveType.TriangleList };
            var destBufferData = new byte[firstVertexBuffer.Declaration.VertexStride * totalVertexCount];
            result.VertexBuffers = new VertexBufferBinding[]
            {
                new VertexBufferBinding(

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