我试图通过模型列中的实体框架lambda创建动态公式
public class OutputModel
{
public decimal Result {get;set;}
}
public class TableTest
{
public decimal A {get;set;}
public decimal B {get;set;}
public decimal C {get;set;}
}
Expression<Func<TableTest, OutputModel>> _expr = t => new OutputModel();
TestExpressionVisitor _visitor = new TestExpressionVisitor();
_visitor.Visit(_expr);
var _result = new TempDataContext().TableTests.Select(_expr);
我正在考虑使用表达式访问者来修改结果
public class TestExpressionVisitor : ExpressionVisitor
{
public override Expression Visit(Expression node)
{
return base.Visit(node);
}
protected override Expression VisitMember(MemberExpression node)
{
return base.VisitMember(node);
}
}
但不确定如何构造一个可以从字符串参数({A} + {B}({C}))中为列执行算术函数的表达式,其中A,B和C来自TableTest并将结果放在OutputModel.Result。我是否正确使用Expressionvisitor?
任何帮助,将不胜感激。
要扩展我的评论,通常您需要:
(A+B)*C
,则令牌将是“Bracket(open),Variable(A),Operator(+),Variable B,Bracket(Close),Operator(*),Variable(C)”对象。
我不会详细介绍如何执行此操作,因为它不是问题的一部分,但我曾经使用正则表达式序列编写了一个非常简单但功能强大的标记生成器,因为这些已经完成了解析所需的大部分艰苦工作。如果是令牌队列,最后一步将是:
// for demo purposes I manually fill the list of tokens
// with the tokens in order how they are output by the shunting-yard algorithm
var tokenQueue = new Token[]
{
new VariableToken("A"),
new VariableToken("B"),
new OperatorToken("+"),
new VariableToken("C"),
new OperatorToken("*")
};
var inputParameter = Expression.Parameter(typeof(TableTest));
var expressions = new Stack<Expression>();
foreach (var token in tokenQueue)
{
// transform token to expression by using the helper methods of https://msdn.microsoft.com/de-de/library/system.linq.expressions.expression_methods(v=vs.110).aspx
switch (token)
{
case VariableToken variableToken:
// this will reference the property in your TableTest input specified by the variable name, e.g. "A" will reference TableTest.A
expressions.Push(Expression.Property(inputParameter, variableToken.Name));
break;
case OperatorToken operatorToken:
// This will take two expression from the stack, give these to input to an operator and put the result back onto the queue for use for the next operator
var rightOperand = expressions.Pop();
var leftOperand = expressions.Pop();
if (operatorToken.Name == "+")
{
expressions.Push(Expression.Add(leftOperand, rightOperand));
}
else if (operatorToken.Name == "*")
{
expressions.Push(Expression.Multiply(leftOperand, rightOperand));
}
break;
}
}
// create and fill output model with final expression
var outputModelExpr = Expression.New(typeof(OutputModel).GetConstructor(new[] {typeof(decimal) }), expressions.Single());
// create the lambda expression
// in this example it will have the form: x => return new OutputModel((x.A + x.B) * x.C)
Expression<Func<TableTest, OutputModel>> lambda = Expression.Lambda<Func<TableTest, OutputModel>>(outputModelExpr, inputParameter);
// only for testing purposes: compile it to a function and run it
var calc = lambda.Compile();
var testInput = new TableTest { A = 1, B = 2, C = 3 };
Console.WriteLine(calc(testInput).Result); // returns 9, because (A + B) * C = (1 + 2) * 3 = 9
使用令牌类:
public abstract class Token
{
public string Name { get; protected set; }
}
public class VariableToken : Token
{
public VariableToken(string name) { Name = name; }
}
public class OperatorToken : Token
{
public OperatorToken(string name) { Name = name; }
}
请注意我向OutputModel添加了一个构造函数,因为这使表达式更容易:
public class OutputModel
{
public OutputModel(decimal result) { Result = result; }
public decimal Result {get;set;}
}