Where exactly are you stuck?
Possibly with the
MpxFraisContext db
parameter. Here you need a "mock". That is, an object of that type which contains some data or none (depending on the singular test). You may create such an object yourself, or use a "mocking framework", e.g. Moq.
A function could look like:
[Test]
public void TestListeSessionFrais()
{
MpxFraisContext db = [create and fill that object with useful data]
int siteID = 1;
int stval = 2;
string codeFrais = "a";
IEnumerable<w_h59mpxsessionfrais> retVal = [YourClass].ListeSessionFrais(db, siteID, stval, codeFrais);
int expectedCount = 1;
Assert.IsNotNull(retVal, "the function must return a value different from null");
}</w_h59mpxsessionfrais>