C# Linq First 和 FirstOrDefault的区别

Enumerable.First 方法

返回序列中的第一个元素。

Enumerable.FirstOrDefault 方法

返回序列中的第一个元素；如果序列中不包含任何元素，则返回默认值。

注： 在使用时，如果返回的是对象， 建议使用FirstOrDefault ， 并对返回的对象进行判空操作

1. FirstOrDefault

如果查询的数据不存在， 则返回 null

namespace ConsoleApplication3
{
    class Program
    {
        static void Main(string[] args)
        {
            var compoundList = new List();
            compoundList.Add(new CompoundType()
            {
                CompoundTypeID = "1",
                TypeEN = "Type1"
            });
            compoundList.Add(new CompoundType()
            {
                CompoundTypeID = "2",
                TypeEN = "Type2"
            });

            var search = compoundList.Where(d => d.CompoundTypeID == "0").FirstOrDefault(); //返回null
            Console.WriteLine("Compound type={0}, typeen = {1}", search.CompoundTypeID, search.TypeEN); // NullException
 
            Console.ReadKey();
        }
    }

    public class CompoundType
    {
        public string CompoundTypeID { get; set; }
        public string TypeEN { get; set; }
    }
}

2. First

如果查询的数据不存在， 则抛System.InvalidOperationException异常

namespace ConsoleApplication3
{
    class Program
    {
        static void Main(string[] args)
        {
            var compoundList = new List();
            compoundList.Add(new CompoundType()
            {
                CompoundTypeID = "1",
                TypeEN = "Type1"
            });
            compoundList.Add(new CompoundType()
            {
                CompoundTypeID = "2",
                TypeEN = "Type2"
            });

            var search = compoundList.Where(d => d.CompoundTypeID == "0").First(); // System.InvalidOperationExceptionan 异常
            Console.WriteLine("Compound type={0}, typeen = {1}", search.CompoundTypeID, search.TypeEN);
 
            Console.ReadKey();
        }
    }

    public class CompoundType
    {
        public string CompoundTypeID { get; set; }
        public string TypeEN { get; set; }
    }
}