using System.IO;
using IxMilia.Dxf;
using IxMilia.Dxf.Entities;
// ...
DxfFile dxfFile;
using (FileStream fs = new FileStream(@"C:\Path\To\File.dxf", FileMode.Open))
{
    dxfFile = DxfFile.Load(fs);
}

foreach (DxfEntity entity in dxfFile.Entities)
{
    switch (entity.EntityType)
    {
        case DxfEntityType.Line:
            DxfLine line = (DxfLine)entity;
            // ...
            break;
        // ...
    }
}

using System.IO;
using IxMilia.Dxf;
using IxMilia.Dxf.Entities;
// ...

DxfFile dxfFile = new DxfFile();
dxfFile.Entities.Add(new DxfLine(new DxfPoint(0, 0, 0), new DxfPoint(50, 50, 0)));
// ...

using (FileStream fs = new FileStream(@"C:\Path\To\File.dxf", FileMode.Open))
{
    dxfFile.Save(fs);
}