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# qp_cs.cs

    /* Copyright 2025, Gurobi Optimization, LLC */

    /* This example formulates and solves the following simple QP model:

         minimize    x^2 + x*y + y^2 + y*z + z^2 + 2 x
         subject to  x + 2 y + 3 z >= 4
                     x +   y       >= 1
                     x, y, z non-negative

       It solves it once as a continuous model, and once as an integer model.
    */

    using System;
    using Gurobi;

    class qp_cs
    {
      static void Main()
      {
        try {
          GRBEnv    env   = new GRBEnv("qp.log");
          GRBModel  model = new GRBModel(env);

          // Create variables

          GRBVar x = model.AddVar(0.0, 1.0, 0.0, GRB.CONTINUOUS, "x");
          GRBVar y = model.AddVar(0.0, 1.0, 0.0, GRB.CONTINUOUS, "y");
          GRBVar z = model.AddVar(0.0, 1.0, 0.0, GRB.CONTINUOUS, "z");

          // Set objective

          GRBQuadExpr obj = x*x + x*y + y*y + y*z + z*z + 2*x;
          model.SetObjective(obj);

          // Add constraint: x + 2 y + 3 z >= 4

          model.AddConstr(x + 2 * y + 3 * z >= 4.0, "c0");

          // Add constraint: x + y >= 1

          model.AddConstr(x + y >= 1.0, "c1");

          // Optimize model

          model.Optimize();

          Console.WriteLine(x.VarName + " " + x.X);
          Console.WriteLine(y.VarName + " " + y.X);
          Console.WriteLine(z.VarName + " " + z.X);

          Console.WriteLine("Obj: " + model.ObjVal + " " + obj.Value);

          // Change variable types to integer

          x.VType = GRB.INTEGER;
          y.VType = GRB.INTEGER;
          z.VType = GRB.INTEGER;

          // Optimize model

          model.Optimize();

          Console.WriteLine(x.VarName + " " + x.X);
          Console.WriteLine(y.VarName + " " + y.X);
          Console.WriteLine(z.VarName + " " + z.X);

          Console.WriteLine("Obj: " + model.ObjVal + " " + obj.Value);

          // Dispose of model and env

          model.Dispose();
          env.Dispose();

        } catch (GRBException e) {
          Console.WriteLine("Error code: " + e.ErrorCode + ". " + e.Message);
        }
      }
    }

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