Error occurred. Collection was modified; enumeration operation might not execute

I have following piece of code in my project "foreach loop" throws the following excepiton "
Error occurred. Collection was modified; enumeration operation might not execute"

foreach (Assessment.tblMitchellLandscapeIDRow MitchellRow in objAssessment.tblMitchellLandscapeID.Rows)
{                                
     if (MitchellRow.AssessmentVersionID == AssessmentVersionID)
     DeleteMitchellLandscape(ref objAssessment, MitchellRow.MitchellID, UserFullname, ref ErrorMessage);
}

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Delete mitchell landscape calls the following  but not sure why the foreach loop throws that exception. Some one please correct me what's the right way to handle this

        public bool DeleteMitchellLandscape(ref Assessment objAssessment, int MitchellIDToDelete, string UserFullname, ref string ErrorMessage)
        {
            string RowFilter = @"MitchellID=" + MitchellIDToDelete;
            Assessment.tblMitchellLandscapeIDRow[] mitchellLandscaperIdRowsows = (Assessment.tblMitchellLandscapeIDRow[])objAssessment.tblMitchellLandscapeID.Select(RowFilter);
            if ((mitchellLandscaperIdRowsows != null) && (mitchellLandscaperIdRowsows.Length != 0))
            {
                if (mitchellLandscaperIdRowsows.Length == 1)
                {
                    if (mitchellLandscaperIdRowsows[0].MitchellID > 0)
                    {
                        mitchellLandscaperIdRowsows[0].UpdatedBySystemUser = UserFullname;
                        mitchellLandscaperIdRowsows[0].SaveType = (int)EnumCollection.SaveType.RemoveOnly;

                    }
                    else
                    {
                        mitchellLandscaperIdRowsows[0].Delete();
                        objAssessment.AcceptChanges();
                    }
                }
                else
                {
                    //Cannot have more than one row with same key
                    ErrorMessage = "Error: More than one record found - Mitchell Landscape ID = " + Convert.ToString(MitchellIDToDelete);
                    return false;
                }
            }
            else
            {
                //Must have at least one row with same key
                ErrorMessage = "Error: Record not found - Mitchell Landscape ID = " + Convert.ToString(MitchellIDToDelete);
                return false;
            }
            return true;
        }

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