如何將表達式<Func <T, object>>轉換爲表達式<Func <object，object >>

Question

有沒有辦法將Expression<Func<T, object>>轉換爲Expression<Func<object, object>>？

Answer 1

我不得不之前做到這一點...

public static class ExpressionHelper { 
    public static Expression<Func<object,object>> ConvertParameterToObject<T>(this Expression<Func<T,object>> source){ 
      return source.ReplaceParametersWithBase<T,object,object>(); 
    } 

    public static Expression<Func<TBase,TResult>> ReplaceParameterWithBase<T,TResult,TBase>(this Expression<Func<T,TResult>> lambda) 
     where T :TBase 
    { 
     var param = lambda.Parameters.Single(); 
     return (Expression<Func<TBase,TResult>>) 
      ParameterRebinder.ReplaceParameters(new Dictionary<ParameterExpression, ParameterExpression> 
               { 
                { param, Expression.Parameter(typeof (TBase), param.Name) } 
               }, lambda.Body); 
    } 
} 


public class ParameterRebinder : ExpressionVisitor 
{ 

    private readonly Dictionary<ParameterExpression, ParameterExpression> map; 



    public ParameterRebinder(Dictionary<ParameterExpression, ParameterExpression> map) 
    { 

     this.map = map ?? new Dictionary<ParameterExpression, ParameterExpression>(); 

    } 



    public static Expression ReplaceParameters(Dictionary<ParameterExpression, ParameterExpression> map, Expression exp) 
    { 

     return new ParameterRebinder(map).Visit(exp); 

    } 



    protected override Expression VisitParameter(ParameterExpression p) 
    { 

     ParameterExpression replacement; 

     if (map.TryGetValue(p, out replacement)) 
     { 

      p = replacement; 

     } 

     return base.VisitParameter(p); 

    } 

} 

Answer 2

怎麼是這樣的：

static Expression<Func<object,object>> ConvertFunction<T>(Expression<Func<T,object>> function)  
{ 
    ParameterExpression p=Expression.Parameter(typeof(object)); 

    return Expression.Lambda<Func<object,object>> 
    (
     Expression.Invoke(function,Expression.Convert(p,typeof(T))), p 
    ); 
} 

那麼你可以說這樣的事情：

Expression<Func<string,object>> foo=s=>s.Length; 
Expression<Func<object,object>> bar=ConvertFunction(foo); 

var call=bar.Compile(); 
Console.Write(call("hello")) ; // Prints 5