You can build an expression tree to perform a for loop by using Expression.Loop method. For example, you have the following code.
int initValue = 0;
int maxValue = 5;
for (int loopVar = initValue; loopVar < maxValue; loopVar++)
{
Console.WriteLine(loopVar);
}
Here is the code that is required to build the same functionality using expression tree.
public static Expression For(ParameterExpression loopVar, Expression initValue, Expression condition, Expression increment, Expression loopContent)
{
var initAssign = Expression.Assign(loopVar, initValue);
var breakLabel = Expression.Label("LoopBreak");
var ifThenElseExpr = Expression.IfThenElse(
condition,
Expression.Block(
loopContent,
increment
),
Expression.Break(breakLabel)
);
var loop = Expression.Loop(ifThenElseExpr, breakLabel);
var block = Expression.Block(new[] { loopVar },
initAssign,
loop
);
return block;
}
This function can be easily used as shown below.
int initValue = 0;
int maxValue = 5;
var loopVar = Expression.Parameter(typeof(int), "loopVar");
var loopBody = Expression.Call(typeof(Console).GetMethod("WriteLine", new[] { typeof(int) }), loopVar);
var initValueExpr = Expression.Constant(initValue);
var incrementExpr = Expression.PostIncrementAssign(loopVar);
var conditionExpr = Expression.LessThan(loopVar, Expression.Constant(maxValue));
var loop = For(loopVar, initValueExpr, conditionExpr, incrementExpr, loopBody);
var result = Expression.Lambda<Action<int>>(loop, loopVar).Compile();
result(initValue);