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我们在开发时，一般都会编写自已的数据库帮助类，如：DbHelper。DbHelper类中可能都有类拟有：一个根据条件查询得到一个实体集的方法:

/// /// 根据条件语句，查实体列表集 /// 调用：如Users实体：字段：ID、Name /// List list = GetListByWhereString (“ Id >0 and Name like ‘FengCodes%’ ”); /// /// 实体类（实体名与数据表的名相同） /// 条件语句（不含where） /// public List GetListByWhereString(string strWhere) where T : class,new() { //根据T获得数据表名 T t = new T(); string tableName= t.GetType().Name; //接着拼接sql语句　string sql=string.format(“select * from {0} ”,tableName); 　If(!string.IsNullOrEmpty(strWhere)) 　{ sql=sql+” where ”+strWhere; 　} //获得sql语句后，就可下一步操作，略 // …… }

实体类：

/// ///测试的实体类 /// public class Users { public int Id{set;get;} public string Name{set;get;} }

至这里，有人会想 strWhere查询语句，又要自已手写拼接条件sql，能不能方便一些的方法呢？如：lambda表达式。

/// /// 根据条件语句，查实体列表集 /// 调用：User实体：字段：ID、Name /// List list = GetList(“ Id >0 and Name like ‘FengCodes%’ ”); /// /// 实体类（实体名与数据表的名相同） /// 条件语句（不含where） /// public List GetList(Expression wherePredicate) where T : class,new() { //有个方法据 wherePredicate的lambda表达式转为 strWhere拼接查询条件 string strWhere=GetWhereByLambda(wherePredicate); //调用上面方法返回 return GetListByWhereString(strWhere); ｝

说到这里,就要入主题：如何把lambda表达式转为where条件sql? 如何实现 GetWhereByLambda方法？

参考了国外网站的一些方法后，结合本构想：可以重写 ExpressionVisitor类来实现!

上代码：

using System; using System.Collections; using System.Collections.Generic; using System.Collections.ObjectModel; using System.Linq; using System.Linq.Expressions; using System.Reflection; using System.Text; namespace LambdaToWhereSqlTest.Common { #region Expression public abstract class ExpressionVisitor35 { protected ExpressionVisitor35() { } protected virtual Expression Visit(Expression exp) { if (exp == null) return exp; switch (exp.NodeType) { case ExpressionType.Negate: case ExpressionType.NegateChecked: case ExpressionType.Not: case ExpressionType.Convert: case ExpressionType.ConvertChecked: case ExpressionType.ArrayLength: case ExpressionType.Quote: case ExpressionType.TypeAs: return this.VisitUnary((UnaryExpression)exp); case ExpressionType.Add: case ExpressionType.AddChecked: case ExpressionType.Subtract: case ExpressionType.SubtractChecked: case ExpressionType.Multiply: case ExpressionType.MultiplyChecked: case ExpressionType.Divide: case ExpressionType.Modulo: case ExpressionType.And: case ExpressionType.AndAlso: case ExpressionType.Or: case ExpressionType.OrElse: case ExpressionType.LessThan: case ExpressionType.LessThanOrEqual: case ExpressionType.GreaterThan: case ExpressionType.GreaterThanOrEqual: case ExpressionType.Equal: case ExpressionType.NotEqual: case ExpressionType.Coalesce: case ExpressionType.ArrayIndex: case ExpressionType.RightShift: case ExpressionType.LeftShift: case ExpressionType.ExclusiveOr: return this.VisitBinary((BinaryExpression)exp); case ExpressionType.TypeIs: return this.VisitTypeIs((TypeBinaryExpression)exp); case ExpressionType.Conditional: return this.VisitConditional((ConditionalExpression)exp); case ExpressionType.Constant: return this.VisitConstant((ConstantExpression)exp); case ExpressionType.Parameter: return this.VisitParameter((ParameterExpression)exp); case ExpressionType.MemberAccess: return this.VisitMemberAccess((MemberExpression)exp); case ExpressionType.Call: return this.VisitMethodCall((MethodCallExpression)exp); case ExpressionType.Lambda: return this.VisitLambda((LambdaExpression)exp); case ExpressionType.New: return this.VisitNew((NewExpression)exp); case ExpressionType.NewArrayInit: case ExpressionType.NewArrayBounds: return this.VisitNewArray((NewArrayExpression)exp); case ExpressionType.Invoke: return this.VisitInvocation((InvocationExpression)exp); case ExpressionType.MemberInit: return this.VisitMemberInit((MemberInitExpression)exp); case ExpressionType.ListInit: return this.VisitListInit((ListInitExpression)exp); default: throw new Exception(string.Format("Unhandled expression type: '{0}'", exp.NodeType)); } } protected virtual MemberBinding VisitBinding(MemberBinding binding) { switch (binding.BindingType) { case MemberBindingType.Assignment: return this.VisitMemberAssignment((MemberAssignment)binding); case MemberBindingType.MemberBinding: return this.VisitMemberMemberBinding((MemberMemberBinding)binding); case MemberBindingType.ListBinding: return this.VisitMemberListBinding((MemberListBinding)binding); default: throw new Exception(string.Format("Unhandled binding type '{0}'", binding.BindingType)); } } protected virtual ElementInit VisitElementInitializer(ElementInit initializer) { ReadOnlyCollection arguments = this.VisitExpressionList(initializer.Arguments); if (arguments != initializer.Arguments) { return Expression.ElementInit(initializer.AddMethod, arguments); } return initializer; } protected virtual Expression VisitUnary(UnaryExpression u) { Expression operand = this.Visit(u.Operand); if (operand != u.Operand) { return Expression.MakeUnary(u.NodeType, operand, u.Type, u.Method); } return u; } protected virtual Expression VisitBinary(BinaryExpression b) { Expression left = this.Visit(b.Left); Expression right = this.Visit(b.Right); Expression conversion = this.Visit(b.Conversion); if (left != b.Left || right != b.Right || conversion != b.Conversion) { if (b.NodeType == ExpressionType.Coalesce && b.Conversion != null) return Expression.Coalesce(left, right, conversion as LambdaExpression); else return Expression.MakeBinary(b.NodeType, left, right, b.IsLiftedToNull, b.Method); } return b; } protected virtual Expression VisitTypeIs(TypeBinaryExpression b) { Expression expr = this.Visit(b.Expression); if (expr != b.Expression) { return Expression.TypeIs(expr, b.TypeOperand); } return b; } protected virtual Expression VisitConstant(ConstantExpression c) { return c; } protected virtual Expression VisitConditional(ConditionalExpression c) { Expression test = this.Visit(c.Test); Expression ifTrue = this.Visit(c.IfTrue); Expression ifFalse = this.Visit(c.IfFalse); if (test != c.Test || ifTrue != c.IfTrue || ifFalse != c.IfFalse) { return Expression.Condition(test, ifTrue, ifFalse); } return c; } protected virtual Expression VisitParameter(ParameterExpression p) { return p; } protected virtual Expression VisitMemberAccess(MemberExpression m) { Expression exp = this.Visit(m.Expression); if (exp != m.Expression) { return Expression.MakeMemberAccess(exp, m.Member); } return m; } protected virtual Expression VisitMethodCall(MethodCallExpression m) { //MethodCallExpression mce = m; //if (mce.Method.Name == "Like") // return string.Format("({0} like {1})", ExpressionRouter(mce.Arguments[0]), ExpressionRouter(mce.Arguments[1])); //else if (mce.Method.Name == "NotLike") // return string.Format("({0} Not like {1})", ExpressionRouter(mce.Arguments[0]), ExpressionRouter(mce.Arguments[1])); //else if (mce.Method.Name == "In") // return string.Format("{0} In ({1})", ExpressionRouter(mce.Arguments[0]), ExpressionRouter(mce.Arguments[1])); //else if (mce.Method.Name == "NotIn") // return string.Format("{0} Not In ({1})", ExpressionRouter(mce.Arguments[0]), ExpressionRouter(mce.Arguments[1])); //MethodCallExpression mce = m; Expression obj = this.Visit(m.Object); IEnumerable args = this.VisitExpressionList(m.Arguments); if (obj != m.Object || args != m.Arguments) { return Expression.Call(obj, m.Method, args); } return m; } protected virtual ReadOnlyCollection VisitExpressionList(ReadOnlyCollection original) { List list = null; for (int i = 0, n = original.Count; i < n; i++) { Expression p = this.Visit(original[i]); if (list != null) { list.Add(p); } else if (p != original[i]) { list = new List(n); for (int j = 0; j < i; j++) { list.Add(original[j]); } list.Add(p); } } if (list != null) { return list.AsReadOnly(); } return original; } protected virtual MemberAssignment VisitMemberAssignment(MemberAssignment assignment) { Expression e = this.Visit(assignment.Expression); if (e != assignment.Expression) { return Expression.Bind(assignment.Member, e); } return assignment; } protected virtual MemberMemberBinding VisitMemberMemberBinding(MemberMemberBinding binding) { IEnumerable bindings = this.VisitBindingList(binding.Bindings); if (bindings != binding.Bindings) { return Expression.MemberBind(binding.Member, bindings); } return binding; } protected virtual MemberListBinding VisitMemberListBinding(MemberListBinding binding) { IEnumerable initializers = this.VisitElementInitializerList(binding.Initializers); if (initializers != binding.Initializers) { return Expression.ListBind(binding.Member, initializers); } return binding; } protected virtual IEnumerable VisitBindingList(ReadOnlyCollection original) { List list = null; for (int i = 0, n = original.Count; i < n; i++) { MemberBinding b = this.VisitBinding(original[i]); if (list != null) { list.Add(b); } else if (b != original[i]) { list = new List(n); for (int j = 0; j < i; j++) { list.Add(original[j]); } list.Add(b); } } if (list != null) return list; return original; } protected virtual IEnumerable VisitElementInitializerList(ReadOnlyCollection original) { List list = null; for (int i = 0, n = original.Count; i < n; i++) { ElementInit init = this.VisitElementInitializer(original[i]); if (list != null) { list.Add(init); } else if (init != original[i]) { list = new List(n); for (int j = 0; j < i; j++) { list.Add(original[j]); } list.Add(init); } } if (list != null) return list; return original; } protected virtual Expression VisitLambda(LambdaExpression lambda) { Expression body = this.Visit(lambda.Body); if (body != lambda.Body) { return Expression.Lambda(lambda.Type, body, lambda.Parameters); } return lambda; } protected virtual NewExpression VisitNew(NewExpression nex) { IEnumerable args = this.VisitExpressionList(nex.Arguments); if (args != nex.Arguments) { if (nex.Members != null) return Expression.New(nex.Constructor, args, nex.Members); else return Expression.New(nex.Constructor, args); } return nex; } protected virtual Expression VisitMemberInit(MemberInitExpression init) { NewExpression n = this.VisitNew(init.NewExpression); IEnumerable bindings = this.VisitBindingList(init.Bindings); if (n != init.NewExpression || bindings != init.Bindings) { return Expression.MemberInit(n, bindings); } return init; } protected virtual Expression VisitListInit(ListInitExpression init) { NewExpression n = this.VisitNew(init.NewExpression); IEnumerable initializers = this.VisitElementInitializerList(init.Initializers); if (n != init.NewExpression || initializers != init.Initializers) { return Expression.ListInit(n, initializers); } return init; } protected virtual Expression VisitNewArray(NewArrayExpression na) { IEnumerable exprs = this.VisitExpressionList(na.Expressions); if (exprs != na.Expressions) { if (na.NodeType == ExpressionType.NewArrayInit) { return Expression.NewArrayInit(na.Type.GetElementType(), exprs); } else { return Expression.NewArrayBounds(na.Type.GetElementType(), exprs); } } return na; } protected virtual Expression VisitInvocation(InvocationExpression iv) { IEnumerable args = this.VisitExpressionList(iv.Arguments); Expression expr = this.Visit(iv.Expression); if (args != iv.Arguments || expr != iv.Expression) { return Expression.Invoke(expr, args); } return iv; } } public class PartialEvaluator : ExpressionVisitor35 { private Func m_fnCanBeEvaluated; private HashSet m_candidates; public PartialEvaluator() : this(CanBeEvaluatedLocally) { } public PartialEvaluator(Func fnCanBeEvaluated) { this.m_fnCanBeEvaluated = fnCanBeEvaluated; } public Expression Eval(Expression exp) { this.m_candidates = new Nominator(this.m_fnCanBeEvaluated).Nominate(exp); return this.Visit(exp); } protected override Expression Visit(Expression exp) { if (exp == null) { return null; } if (this.m_candidates.Contains(exp)) { return this.Evaluate(exp); } return base.Visit(exp); } private Expression Evaluate(Expression e) { if (e.NodeType == ExpressionType.Constant) { return e; } LambdaExpression lambda = Expression.Lambda(e); Delegate fn = lambda.Compile(); return Expression.Constant(fn.DynamicInvoke(null), e.Type); } private static bool CanBeEvaluatedLocally(Expression exp) { return exp.NodeType != ExpressionType.Parameter; } #region Nominator /// /// Performs bottom-up analysis to determine which nodes can possibly /// be part of an evaluated sub-tree. /// private class Nominator : ExpressionVisitor35 { private Func m_fnCanBeEvaluated; private HashSet m_candidates; private bool m_cannotBeEvaluated; internal Nominator(Func fnCanBeEvaluated) { this.m_fnCanBeEvaluated = fnCanBeEvaluated; } internal HashSet Nominate(Expression expression) { this.m_candidates = new HashSet(); this.Visit(expression); return this.m_candidates; } protected override Expression Visit(Expression expression) { if (expression != null) { bool saveCannotBeEvaluated = this.m_cannotBeEvaluated; this.m_cannotBeEvaluated = false; base.Visit(expression); if (!this.m_cannotBeEvaluated) { if (this.m_fnCanBeEvaluated(expression)) { this.m_candidates.Add(expression); } else { this.m_cannotBeEvaluated = true; } } this.m_cannotBeEvaluated |= saveCannotBeEvaluated; } return expression; } } #endregion } internal class ConditionBuilder : ExpressionVisitor35 { /// /// 数据库类型 /// private string m_dataBaseType = ""; /// /// 字段是否加引号 /// private bool m_ifWithQuotationMarks = false; private List m_arguments; private Stack m_conditionParts; public string Condition { get; private set; } public object[] Arguments { get; private set; } #region 加双引号 /// /// 加双引号 /// /// 字串 /// public static string AddQuotationMarks(string str) { if (str != null) { return "\"" + str.Trim() + "\""; } else { return str; } } #endregion #region 设置是否加双引号 public void SetIfWithQuotationMarks(bool ifwith) { m_ifWithQuotationMarks = ifwith; } #endregion #region 设置是数据库类型 public void SetDataBaseType(string databaseType) { if(!string.IsNullOrEmpty(databaseType)) { m_dataBaseType = databaseType; } } #endregion public void Build(Expression expression) { PartialEvaluator evaluator = new PartialEvaluator(); Expression evaluatedExpression = evaluator.Eval(expression); this.m_arguments = new List(); this.m_conditionParts = new Stack(); this.Visit(evaluatedExpression); this.Arguments = this.m_arguments.ToArray(); this.Condition = this.m_conditionParts.Count > 0 ? this.m_conditionParts.Pop() : null; } protected override Expression VisitBinary(BinaryExpression b) { if (b == null) return b; string opr; switch (b.NodeType) { case ExpressionType.Equal: opr = "="; break; case ExpressionType.NotEqual: opr = ""; break; case ExpressionType.GreaterThan: opr = ">"; break; case ExpressionType.GreaterThanOrEqual: opr = ">="; break; case ExpressionType.LessThan: opr = "="; case ExpressionType.LessThan: return "

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