using an Array type as a Parameter Expression

Question

When declaring a parameter expression, for example:

ParameterExpression x = Expression.Parameter(typeof(double),"x");

You can then use this parameter to construct Lambda expressions:

Expression Lowerbound = Expression.Constant(0.0,typeof(double));

Expression GreaterThanorEqual = Expression.GreaterThanOrEqual(x, Lowerbound);

Expression TestExpression = Expression.Lambda(GreaterThanorEqual,x);

Console.WriteLine(TestExpression.ToString());

    // returns x => (x >= 0)

I now need a way to construct expressions of the form

x => (x[0] >= 0)
x => (x[1] >= 0)
x => (x[2] >= 0)
... et cetera

I cannot, however, find a way to define a parameter

Expression.Parameter(typeof(double[]),"x");

array as one Parameter

and then

Expression.Parameter(typeof(double),"x["+i+"]");

As another

is there a way to define a Parameter that has a Type of double[] ?

Your Parameter call should work, are you asking how to construct the index expression? Expression.ArrayAccess(xParam, Expression.Constant(0)) should work. — Lee
– Lee, Commented Aug 21, 2017 at 16:58

joncloud · Accepted Answer · 2017-08-21 17:01:09Z

In order to access an array during the expression use Expression.ArrayIndex. This takes two parameters: 1) the array to get an item from, 2) the index to pass into the array.

The following code should compile the expression into a delegate, which takes double[] array, and int index in order to perform the x => x[i] >= 0 expression.

var arrayParam = Expression.Parameter(typeof(double[]), "x");
var indexParam = Expression.Parameter(typeof(int), "i");

var left = Expression.ArrayIndex(arrayParam, indexParam);
var right = Expression.Constant(0.0, typeof(double));

var body = Expression.GreaterThanOrEqual(left, right);

// Create and compile a lambda expression into a delegate to perform:
// x => x[i] >= 0
var func = Expression.Lambda<Func<double[], int, bool>>(body, arrayParam, indexParam)
    .Compile();

double[] data = new double[] { -2.0, -1.0, 0.0, 1.0, 2.0, 3.0 };

for (int i = 0; i < data.Length; i++)
{
    var d = data[i];
    var x = func(data, i);
    Console.WriteLine($"i: {d} => {x}");
}

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