将Expression转换为Expression.Lambda <Func<object, bool> &gt;

Question

I have a method that builds an expression tree, based on the Type of the object that is passed to the method. Once the tree is built, I want to convert it and return it with the return type as is shown below.

public static Expression<Func<object, bool>> BuildExpression(Type type, ...)
{
    // build the expression...
    ParameterExpression param = Expression.Parameter(type, "m");
    Expression expression = null;

    // simplified version of building the expression tree
    MemberExpression member = Expression.Property(param, filter.Property);
    ConstantExpression constant = Expression.Constant(filter.Value);
    expression = Expression.Equal(member, constant);

   // ...

   // IT FAILS ON THIS LINE!!!
   return Expression.Lambda<Func<object, bool>>(expression, param);
}

I've looked at a few conversion answers, but to no avail. Any advice?

Answer 1

Here is your code with modifications as described in my previous comment.

1) Your function returns expression that describes function with single argument. And this argument is of type Object . So you should use Object type when creating parameter "m" expression.

2) Before accessing the property parameter should be cast back to desired type. See Expression.Convert .

public static Expression<Func<object, bool>> BuildExpression(Type type, ...)
{
    // build the expression...
    ParameterExpression param = Expression.Parameter(typeof(Object), "m");
    Expression expression = null;

    UnaryExpression convert = Expression.Convert(param, type);

    // simplified version of building the expression tree
    MemberExpression member = Expression.Property(convert, filter.Property);
    ConstantExpression constant = Expression.Constant(filter.Value);
    expression = Expression.Equal(member, constant);

    // ...

    return Expression.Lambda<Func<object, bool>>(expression, param);
}