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[英]Are there any performance considerations to using ExpressionVisitor?
[英]Create Arithmetic Formula using ExpressionVisitor
我正在嘗試從模型的列通過實體框架lambda創建動態公式
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);
我當時在考慮使用表達式訪問者來修改結果
public class TestExpressionVisitor : ExpressionVisitor
{
public override Expression Visit(Expression node)
{
return base.Visit(node);
}
protected override Expression VisitMember(MemberExpression node)
{
return base.VisitMember(node);
}
}
但不是很確定如何構造一個表達式,該表達式可以從字符串參數( {A} + {B}({C}) )進行列的算術運算,其中A,B和C來自TableTest,並將結果放在OutputModel.Result 。 我在使用Expressionvisitor的正確路徑上嗎?
任何幫助,將不勝感激。
要擴展我的評論,通常需要:
(A+B)*C
則標記將是對象“括號(打開),變量(A),運算符(+),變量B,括號(關閉),運算符(*),變量(C )”。 在令牌隊列的情況下,最后一步是:
// 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
使用令牌類:
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; }
}
請注意,我向OutputModel添加了一個構造函數,因為這使表達式更加容易:
public class OutputModel
{
public OutputModel(decimal result) { Result = result; }
public decimal Result {get;set;}
}
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