public static Expression ReplaceParameters([NotNull] Dictionary <ParameterExpression, Expression> map,
[NotNull] Expression exp)
{
var result = new ExpressionParameterRebinder(map).Visit(exp);
return(result ?? exp);
}
/// <summary>
/// Compose two <c>Expression</c> int one by merge function.
/// </summary>
/// <typeparam name="TExpression">Type of <c>Expression</c>.</typeparam>
/// <param name="left">Base first <c>Expression</c>.</param>
/// <param name="right">Base second <c>Expression</c>.</param>
/// <param name="merge">Merge function.</param>
/// <returns>
/// Lambda <c>Expression</c> created with <paramref name="left" />, <paramref name="right" /> <c>Expressions</c>
/// and <paramref name="merge" /> function.
/// </returns>
/// <example>
/// <code>
/// Expression<Func<string, bool>> baseExp1 = str => string.IsNullOrEmpty(str);
/// Expression<Func<string, bool>> baseExp2 = s => s.Length > 0;
/// Expression newExp = ExpressionComposer.Compose(baseExp1, baseExp2, Expression.AndAlso);
/// // str => string.IsNullOrEmpty(str) AndAlso (str.Length > 0)
/// </code>
/// </example>
/// <exception cref="ArgumentNullException">
/// Thrown when <paramref name="left" />, <paramref name="right" />
/// or <paramref name="merge" /> is null.
/// </exception>
internal static Expression <TExpression> Compose <TExpression>(Expression <TExpression> left,
Expression <TExpression> right,
Func <Expression, Expression, Expression> merge)
{
left = left ?? throw new ArgumentNullException(nameof(left));
right = right ?? throw new ArgumentNullException(nameof(right));
merge = merge ?? throw new ArgumentNullException(nameof(merge));
// build parameter map (from parameters of right to parameters of left)
var map = left.Parameters.Select((f, i) => new { f, s = right.Parameters[i] })
.ToDictionary(p => p.s, p => (Expression)p.f);
// replace parameters in the right lambda expression with parameters from the left
var secondBody = ExpressionParameterRebinder.ReplaceParameters(map, right.Body);
// apply composition of lambda expression bodies to parameters from the left expression
return(Expression.Lambda <TExpression>(merge(left.Body, secondBody), left.Parameters));
}