Building Expression Trees

问题

I'm struggling with the idea of how to build an expression tree for more lambdas such as the one below, let alone something that might have multiple statements. For example:

Func<double?, byte[]> GetBytes
      = x => x.HasValue ? BitConverter.GetBytes(x.Value) : new byte[1] { 0xFF };

I would appreciate any thoughts.

回答1:

I would suggest reading through the list of methods on the Expression class, all of your options are listed there, and the Expression Trees Programming Guide.

As for this particular instance:

/* build our parameters */
var pX = Expression.Parameter(typeof(double?));

/* build the body */
var body = Expression.Condition(
    /* condition */
    Expression.Property(pX, "HasValue"),
    /* if-true */
    Expression.Call(typeof(BitConverter),
                    "GetBytes",
                    null, /* no generic type arguments */
                    Expression.Member(pX, "Value")),
    /* if-false */
    Expression.Constant(new byte[] { 0xFF })
);

/* build the method */
var lambda = Expression.Lambda<Func<double?,byte[]>>(body, pX);

Func<double?,byte[]> compiled = lambda.Compile();

来源：https://stackoverflow.com/questions/6273788/building-expression-trees