C# Linq First 和 FirstOrDefault的区别

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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<CompoundType>();            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<CompoundType>();            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; }    }}

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