Unable to convert lambda expressions to delegate type

In order not to litter the topic, I act on the principle of a separate issue-a separate topic

public static double reverstrans(double U, double anguarFrequency, double T) { NumericalIntegration integration = new NumericalIntegration(); Complex u1 = U * integration.Calculate(t => Math.Cos(anguarFrequency), 0, T);//U'(w) Complex u2 = U * integration.Calculate(t => Math.Sin(anguarFrequency), 0, T);//U''(w) Complex img = Complex.ImaginaryOne.Imaginary;// инициализируем новый экземпляр объекта complex samvalue(1,0) Complex ValFurieTrans = u1 - img * u2; //Вычисляем U(w)e^-j*fi(w)= U'(w) - U''(w) return (1 / (2 * Math.PI)) * GetSpectralDensity(U, anguarFrequency, T) * integration.Calculate1(t => Complex.Cos(anguarFrequency * t - ValFurieTrans), 0, T) + Complex.ImaginaryOne * (1 / (2 * Math.PI)) * GetSpectralDensity(U, anguarFrequency, T) * integration.Calculate1(t => Complex.Sin(anguarFrequency * t - ValFurieTrans), 0, T); } 

I changed the code and added Complex.Cos and Complex.Sin but there was a problem with the lambda expression. In the line with return he writes where C is omplex.Cos и Complex.Sin

Cannot convert lambda expression to delegate type "System.Func" because some return types in the block are not implicitly convertible to the delegate return type

WHAT I HAVE BEEN DONE

Given that I have Calculate methods (they are double) and I really need them. I decided to create a separate Calculate for complex numbers. But even then there were big problems.

 interface NumericalIntegrationRule { double Calculate(Func<double, double> func, double min, double max, double subintervalsCount); Complex Calculate1(Func<double, Complex> func, double min, double max, double subintervalsCount); } class NumericalIntegration { public readonly NumericalIntegrationRule DefaultRule = null; public readonly int DefaultSubintervalsCount = 100;//100000 public NumericalIntegration() { DefaultRule = new TrapezoidalRule(); } public NumericalIntegration(NumericalIntegrationRule defaultRule) { DefaultRule = defaultRule; } public NumericalIntegration(NumericalIntegrationRule defaultRule, int defaultSubintervalsCount) : this(defaultRule) { DefaultSubintervalsCount = defaultSubintervalsCount; } public double Calculate(NumericalIntegrationRule rule, Func<double, double> func, double min, double max, double subintervalsCount) { return rule.Calculate(func, min, max, subintervalsCount);--- Неявное преобразование типа "System.Numerics.Complex" в "double" невозможно } public double Calculate(Func<double, double> func, double min, double max) { return Calculate(DefaultRule, func, min, max, DefaultSubintervalsCount); } public double Calculate1(NumericalIntegrationRule rule, Func<double, Complex> func, double min, double max, double subintervalsCount) { return rule.Calculate1(func, min, max, subintervalsCount); } public double Calculate1(Func<double, Complex> func, double min, double max) { return Calculate1(DefaultRule, func, min, max, DefaultSubintervalsCount); } } class TrapezoidalRule : NumericalIntegrationRule { public double Calculate(Func<double,double> func, double min, double max, double subintervalsCount) { double result = 0; double step = (max - min) / subintervalsCount; for (double i = min; i < max; i += step) result += ((func(i) + func(i + step)) / 2) * step; return result; } public Complex Calculate1(Func<double, Complex> func, double min, double max, double subintervalsCount)---"MainFormProg1.FurierTransformReverse.TrapezoidalRule.Calculate1(System.Func<double,System.Numerics.Complex>, double, double, double)": не все ветви кода возвращают значение { Complex result = 0; double step = (max - min) / subintervalsCount; for (double i = min; i < max; i += step) return result += ((func(i) + func(i + step)) / 2) * step; } }