c# – 如何从Property Name创建Expression> expression

x => x.PropertyName

从头开始,像这样：

// Get a reference to the property
var propertyInfo = ExpressionHelper.GetPropertyInfo<TModel>(propertyName);
var model = ExpressionHelper.Parameter<TModel>();

// Build the LINQ expression tree backwards:
// x.Prop
var key = ExpressionHelper.GetPropertyExpression(model,propertyInfo);
// x => x.Prop
var keySelector = ExpressionHelper.GetLambda(typeof(TModel),propertyInfo.PropertyType,model,key);

为了使代码更具可读性,将细节表达式树操作移入此辅助类：

public static class ExpressionHelper
{
    private static readonly MethodInfo LambdaMethod = typeof(Expression)
        .GetMethods()
        .First(x => x.Name == "Lambda" && x.ContainsGenericParameters && x.GetParameters().Length == 2);

    private static MethodInfo GetLambdaFuncBuilder(Type source,Type dest)
    {
        var predicateType = typeof(Func<,>).MakeGenericType(source,dest);
        return LambdaMethod.MakeGenericMethod(predicateType);
    }

    public static PropertyInfo GetPropertyInfo<T>(string name)
        => typeof(T).GetProperties()
        .Single(p => p.Name == name);

    public static ParameterExpression Parameter<T>()
        => Expression.Parameter(typeof(T));

    public static MemberExpression GetPropertyExpression(ParameterExpression obj,PropertyInfo property)
        => Expression.Property(obj,property);

    public static LambdaExpression GetLambda<TSource,TDest>(ParameterExpression obj,Expression arg)
        => GetLambda(typeof(TSource),typeof(TDest),obj,arg);

    public static LambdaExpression GetLambda(Type source,Type dest,ParameterExpression obj,Expression arg)
    {
        var lambdaBuilder = GetLambdaFuncBuilder(source,dest);
        return (LambdaExpression)lambdaBuilder.Invoke(null,new object[] { arg,new[] { obj } });
    }
}

从头开始构建表达式树使您在创建lambda表达式时具有最大的灵活性.根据目标属性类型,它可能不总是表达式< Func< TModel,string>> – 最后一种类型可以是int或其他类型.无论目标属性类型如何,此代码都将构建正确的表达式树.