LINQ

IEnumerable、IEnumerator

这两个接口位于System.Collections.Generic命名空间下。
IEnumrable
源码：https://referencesource.microsoft.com/#mscorlib/system/collections/ienumerable.cs,9be451ac13d86a97

[Guid("496B0ABE-CDEE-11d3-88E8-00902754C43A")]
[System.Runtime.InteropServices.ComVisible(true)]
public interface IEnumerable
{
        // Interfaces are not serializable
        // Returns an IEnumerator for this enumerable Object.  The enumerator provides
        // a simple way to access all the contents of a collection.
        [Pure]
        [DispId(-4)]
        IEnumerator GetEnumerator();
}

实际上就一个方法：IEnumerator GetEnumerator();

IEnumerator
源码：

public interface IEnumerator
    {
        // Interfaces are not serializable
        // Advances the enumerator to the next element of the enumeration and
        // returns a boolean indicating whether an element is available. Upon
        // creation, an enumerator is conceptually positioned before the first
        // element of the enumeration, and the first call to MoveNext 
        // brings the first element of the enumeration into view.
        // 
        bool MoveNext();
    
        // Returns the current element of the enumeration. The returned value is
        // undefined before the first call to MoveNext and following a
        // call to MoveNext that returned false. Multiple calls to
        // GetCurrent with no intervening calls to MoveNext 
        // will return the same object.
        // 
        Object Current {
            get; 
        }
    
        // Resets the enumerator to the beginning of the enumeration, starting over.
        // The preferred behavior for Reset is to return the exact same enumeration.
        // This means if you modify the underlying collection then call Reset, your
        // IEnumerator will be invalid, just as it would have been if you had called
        // MoveNext or Current.
        //
        void Reset();
    }

很明显，如果一个类实现了这两个方法，就能很方便的遍历该类所包含的一些数据。为了使用更加方便，c#更是用foreach关键字来简化使用。

var

匿名类型（隐式类型），就是让编译器猜测数据类型。

var i = new int[] {1,2,3}; //编译器能猜出来`i`是个int数组。

匿名类

var i = new {name = "kun",age ="20"};

我们并没有定义一个类，但是使用的时候直接i.name就能得到kun字符串。

lambda表达式

将函数的声明类型、return等关键字优化掉
(t)=>console.writline(t.tostring());

扩展方法

以Where方法为例，where方法是命名空间System.Linq下public static class Enumerable的方法，注意：Enumerable类并不是IEnumerable接口的实现。Enumerable类下提供一组用于查询实现 IEnumerable 的对象的 static方法。也就是说Enumerable类下的方法都实现了IEnumerable接口。

public override IEnumerable<TSource> Where(Func<TSource, bool> predicate) {
                return new WhereEnumerableIterator<TSource>(source, CombinePredicates(this.predicate, predicate));
            }

class WhereEnumerableIterator<TSource> : Iterator<TSource>
        {
            IEnumerable<TSource> source;
            Func<TSource, bool> predicate;
            IEnumerator<TSource> enumerator;
 
            public WhereEnumerableIterator(IEnumerable<TSource> source, Func<TSource, bool> predicate) {
                this.source = source;
                this.predicate = predicate;
            }
 
            public override Iterator<TSource> Clone() {
                return new WhereEnumerableIterator<TSource>(source, predicate);
            }
 
            public override void Dispose() {
                if (enumerator is IDisposable) ((IDisposable)enumerator).Dispose();
                enumerator = null;
                base.Dispose();
            }
 
            public override bool MoveNext() {
                switch (state) {
                    case 1:
                        enumerator = source.GetEnumerator();
                        state = 2;
                        goto case 2;
                    case 2:
                        while (enumerator.MoveNext()) {
                            TSource item = enumerator.Current;
                            if (predicate(item)) {  //在这里使用传入的lambda表达式筛选下一个item
                                current = item;
                                return true;
                            }
                        }
                        Dispose();
                        break;
                }
                return false;
            }
 
            public override IEnumerable<TResult> Select<TResult>(Func<TSource, TResult> selector) {
                return new WhereSelectEnumerableIterator<TSource, TResult>(source, predicate, selector);
            }
 
            public override IEnumerable<TSource> Where(Func<TSource, bool> predicate) {
                return new WhereEnumerableIterator<TSource>(source, CombinePredicates(this.predicate, predicate));
            }
        }

表达式树

namespace SqlSugar
{
    public class JoinQueryInfo
    {
        public JoinType JoinType{ get; set; }
        public string TableName { get; set; }
        public string ShortName { get; set; }
        public int JoinIndex { get; set; }
        public string JoinWhere { get; set; }
    }
}