Code.cs

using System;
using MathNet.Numerics.LinearAlgebra;
using Microsoft.VisualStudio.TestTools.UnitTesting;

namespace Testing
{
    [TestClass]
    public class UnitTest1
    {
        [TestMethod]
        public void TestMethod1()
        {
            Matrix<float> a = Matrix<float>.Build.Dense(2, 2, new float[] { 10, 19, 25, 10 });

            Console.WriteLine("Det {0}", a.Determinant());
            Console.WriteLine("Det Int {0}", (int)a.Determinant());
            Console.WriteLine("Det Trunc {0}", Math.Truncate(a.Determinant()));
            Console.WriteLine("Det Round {0}", Math.Round(a.Determinant()));
            Console.WriteLine("Det floor {0}", Math.Floor(a.Determinant()));
        }
    }
}

Results

Run:
Det -375
Det Int -374
Det Trunc -374
Det Round -375
Det floor -375

Debug:
Det -375
Det Int -375
Det Trunc -375
Det Round -375
Det floor -376