The triangles are defined using a vertex list (see Vertices) and an index list (see Indices). (NumberOfTriangles is Indices.Count / 3. It is allowed to manipulate the vertex and index lists directly, e.g. a triangle can be added by adding 3 indices to Indices.

Vertex welding: The Add(Triangle, Boolean) method and its overloads can be used to add a new triangles to the mesh. These methods can perform brute-force vertex welding to remove duplicate vertices. The brute-force approach is fast if the mesh has only a small number of vertices.

// Building a simple triangle mesh using brute-force vertex welding.
TriangleMesh mesh = new TriangleMesh();

// Add triangles:
mesh.AddTriangle(new Triangle(v0, v1, v2), true);
mesh.AddTriangle(new Triangle(v4, v1, v0), true);
...

But vertex welding using brute-force is slow if there is a large number of vertices. Therefore, when constructing complex meshes the method WeldVertices(Single) should be used. WeldVertices(Single) implements a fast vertex welding algorithm that handles large meshes efficiently. (However, this algorithm is more resource intensive than the brute-force approach.)

// Building a complex triangle mesh using intelligent vertex welding.
TriangleMesh mesh = new TriangleMesh();

// Add triangles:
mesh.AddTriangle(new Triangle(v0, v1, v2), false);
mesh.AddTriangle(new Triangle(v4, v1, v0), false);
...

// After all vertices are added, remove duplicates.
mesh.WeldVertices(0.001f);

Reference